feat: support custom registries (#18)

Closes #15

CHANGELOG.md

@@ -5,6 +5,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Added
+- Support alternative registries with the `registry` and `index` input
+  parameters.
+
 ### Changed
 - Crate versions are fetched from the sparse index instead of the crates.io
   API.

README.md

@@ -13,7 +13,8 @@ and automatically caches the resulting binaries to speed up subsequent builds.
 | **v2.0:** This major update introduces some breaking changes. Read the [changelog] before updating. |
 
 ## Features
-- Install any Rust binary crate from [crates.io] or a git repository.
+- Install any Rust binary crate from [crates.io], a git repository or an
+  alternate registry.
 - Automatically cache installed binaries to avoid compiling them each run.
 - Keep crates updated, with an optional version range to avoid breakages.
 - Works on Linux, Windows and MacOS runners.
@@ -65,6 +66,16 @@ unexpected dependencies updates.
 `branch`, `tag` and `commit`/`rev` are mutually exclusive. If none of them are
 specified, the latest commit of the default branch will be used.
 
+#### Alternate registry parameters
+- `registry`: Registry name from the Cargo configuration. See
+  [Using an alternate registry](https://doc.rust-lang.org/nightly/cargo/reference/registries.html#using-an-alternate-registry)
+  on the Cargo Book.
+- `index`: URL of the registry index.
+
+`registry` and `index` are mutually exclusive. Only sparse `index` support
+version range resolution, you'll need to specify an exact version when using
+`registry` or a non-sparse `index`.
+
 ### Outputs
 - `version`: The version of the crate that has been installed.
 - `cache-hit`: A boolean indicating whether the crate was restored from cache.

action.yml

@@ -19,11 +19,17 @@ inputs:
     description: "Additional key added to the automatic cache key."
     required: false
 
-  # Crates.io installation
+  # Registry installation
   version:
     description: "Version of the crate to install."
     required: true
     default: "latest"
+  registry:
+    description: "Registry to install the crate from."
+    required: false
+  index:
+    description: "Registry index to install the crate from."
+    required: false
 
   # Git installation
   git: