Fix scheduled WASM extra-platforms job selection (#134613)

## Problem

The scheduled `runtime-extra-platforms` pipeline includes this child
template only for rolling builds, but since #133068 the child has
repeated that check through `variables.isRollingBuild`. That ambient
variable is unavailable during nested compile-time expansion, so it
expands empty and the child selects the WASM-specific branches instead.
Build 1610154 consequently omitted `LibraryTestsCoreCLR_R2R` and
`TestWasmReadyToRun` while adding `SingleThreaded_BuildOnly` and
non-extra browser CoreCLR jobs.

## Fix

Use the explicit `isExtraPlatformsBuild` parameter to select the
scheduled extra-platforms branch and its inverse for WASM-specific
pipeline invocations. Use the explicit `isWasmOnlyBuild` parameter for
WASI smoke `alwaysRun` behavior.

## Validation

- parsed
`eng/pipelines/extra-platforms/runtime-extra-platforms-wasm.yml`
successfully as YAML
- authenticated Azure DevOps definition 154 preview against the PR merge
ref includes `LibraryTestsCoreCLR_R2R` and `TestWasmReadyToRun`
- preview includes both browser smoke AOT jobs, both WASI AOT jobs, and
both browser high-resource AOT jobs
- preview contains no `SingleThreaded_BuildOnly` references and no
non-extra browser CoreCLR build-only jobs
- no pipeline run was queued; validation used the preview API only

`/azp run runtime-extra-platforms` cannot validate this scheduled path
on a PR: PR runs set `Build.Reason` to `PullRequest`, which makes the
root `isRollingBuild` gate omit the WASM child template entirely. The
preview API expands definition 154 against the PR merge ref in the
required rolling context without queuing a build. The next daily
scheduled run after merge will provide the end-to-end confirmation.

> [!NOTE]
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Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 541a2c0a-365a-40d8-adfc-20e687d223b8

Author: radekdoulik

eng/pipelines/extra-platforms/runtime-extra-platforms-wasm.yml

@@ -18,11 +18,11 @@ parameters:
 jobs:
 
 #
-# ************ Rolling builds: *************
+# ************ runtime-extra-platforms invocation: *************
 # - only run full or expensive library-test configurations
 # - rest are covered by runtime
 #
-- ${{ if eq(variables.isRollingBuild, true) }}:
+- ${{ if eq(parameters.isExtraPlatformsBuild, true) }}:
   # Full trimmed CoreCLR ReadyToRun library tests
   - template: /eng/pipelines/common/templates/wasm-coreclr-library-tests.yml
     parameters:
@@ -80,11 +80,11 @@ jobs:
       useHelixMonitor: ${{ parameters.useHelixMonitor }}
 
 #
-# ********** For !rolling builds, IOW - PR builds *************
+# ********** WASM-specific pipeline invocations: *************
 # - run everything, if relevant paths changed
 # - For runtime-wasm, force run all the jobs
 #
-- ${{ if and(ne(variables.isRollingBuild, true), ne(parameters.excludeLibTests, true), ne(parameters.debuggerTestsOnly, true)) }}:
+- ${{ if and(ne(parameters.isExtraPlatformsBuild, true), ne(parameters.excludeLibTests, true), ne(parameters.debuggerTestsOnly, true)) }}:
   # Library tests
   # these run on runtime also
   - template: /eng/pipelines/common/templates/wasm-library-tests.yml
@@ -205,12 +205,12 @@ jobs:
       # ignore test failures for runtime-extra-platforms, but not when this
       # is run as part of a wasm specific pipeline like runtime-wasm
       shouldContinueOnError: ${{ not(parameters.isWasmOnlyBuild) }}
-      alwaysRun: ${{ variables.isRollingBuild }}
+      alwaysRun: ${{ parameters.isWasmOnlyBuild }}
       useHelixMonitor: ${{ parameters.useHelixMonitor }}
       scenarios:
         - WasmTestOnWasmtime
 
-- ${{ if and(ne(variables.isRollingBuild, true), ne(parameters.excludeNonLibTests, true), ne(parameters.debuggerTestsOnly, true)) }}:
+- ${{ if and(ne(parameters.isExtraPlatformsBuild, true), ne(parameters.excludeNonLibTests, true), ne(parameters.debuggerTestsOnly, true)) }}:
   # Builds only
   - template: /eng/pipelines/common/templates/wasm-build-only.yml
     parameters:
@@ -289,7 +289,7 @@ jobs:
       isWasmOnlyBuild: ${{ parameters.isWasmOnlyBuild }}
       useHelixMonitor: ${{ parameters.useHelixMonitor }}
 
-- ${{ if and(ne(variables.isRollingBuild, true), ne(parameters.excludeOptional, true)) }}:
+- ${{ if and(ne(parameters.isExtraPlatformsBuild, true), ne(parameters.excludeOptional, true)) }}:
   - template: /eng/pipelines/common/templates/wasm-library-tests.yml
     parameters:
       platforms: