Add CircleCI command that checks a project's environment variables (#…

…1752)

Related tiny-pilot/tinypilot-pro#1229

In an effort to reduce repetition on our CircleCI config, this PR
includes the following:
* Ports our [environment variable check from Tinypilot
Pro](https://github.com/tiny-pilot/tinypilot-pro/blob/bddd86133a6685cef3ddc43631359ee97181d509/.circleci/config.yml#L273-L285).
    - Less one environment variable we don't use in Community.
* Introduces the env check as a reusable CircleCI command.
* We do this because in pro we make use of the env check more than once.
1.
https://github.com/tiny-pilot/tinypilot-pro/blob/bddd86133a6685cef3ddc43631359ee97181d509/.circleci/config.yml#L273-L285
2.
https://github.com/tiny-pilot/tinypilot-pro/blob/bddd86133a6685cef3ddc43631359ee97181d509/.circleci/config.yml#L335-L347

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.circleci/config.yml

@@ -9,6 +9,23 @@ executors:
   node:
     docker:
       - image: cimg/node:18.16.1
+commands:
+  check_project_env_vars:
+    steps:
+      - run:
+          name: Check for environment variables
+          # These environment variables can either come from the CircleCI
+          # project settings or from the CircleCI context. The latter takes
+          # precedence.
+          # See: https://circleci.com/docs/env-vars/#order-of-precedence
+          command: |
+            if [[ -z "${BACKBLAZE_KEY}"
+               || -z "${BACKBLAZE_KEY_ID}"
+               || -z "${UPLOAD_BUCKET}"
+            ]]; then
+              >&2 echo 'Missing environment variables'
+              exit 1
+            fi
 jobs:
   check_whitespace:
     executor: ubuntu
@@ -226,6 +243,7 @@ jobs:
     environment:
       UPLOAD_PREFIX: community
     steps:
+      - check_project_env_vars
       - checkout
       - attach_workspace:
           at: ./