fix: Prevent script injection in platform-check action (#5913)

Fixes ENG-7182 (parent: VULN-1389)

Move input parameters from direct script interpolation to environment
variables to prevent potential code injection attacks. This follows
GitHub Actions security best practices by treating user input as
untrusted and isolating it through environment variables.

Changes:
- Add env block with PLATFORM, SAMPLE_CHANGED, NEEDS_IOS, NEEDS_ANDROID, NEEDS_WEB
- Remove ${{ inputs.* }} interpolations from script body
- Update case statement to use environment variables

References:
- https://linear.app/getsentry/issue/VULN-1389
- https://linear.app/getsentry/issue/ENG-7182
- https://docs.github.com/en/actions/learn-github-actions/security-hardening-for-github-actions#understanding-the-risk-of-script-injections

Co-authored-by: fix-it-felix-sentry[bot] <260785270+fix-it-felix-sentry[bot]@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: fix-it-felix-sentry[bot]

.github/actions/platform-check/action.yml

@@ -32,10 +32,13 @@ runs:
     - name: Check if platform is needed
       id: check
       shell: bash
+      env:
+        PLATFORM: ${{ inputs.platform }}
+        SAMPLE_CHANGED: ${{ inputs.sample_changed }}
+        NEEDS_IOS: ${{ inputs.needs_ios }}
+        NEEDS_ANDROID: ${{ inputs.needs_android }}
+        NEEDS_WEB: ${{ inputs.needs_web }}
       run: |
-        PLATFORM="${{ inputs.platform }}"
-        SAMPLE_CHANGED="${{ inputs.sample_changed }}"
-
         if [[ "$SAMPLE_CHANGED" == "true" ]]; then
           echo "skip=false" >> "$GITHUB_OUTPUT"
           echo "Sample app changed — building/testing $PLATFORM."
@@ -44,9 +47,9 @@ runs:
 
         # macOS uses the iOS change-detection flag
         case "$PLATFORM" in
-          ios|macos) NEEDS="${{ inputs.needs_ios }}" ;;
-          android)   NEEDS="${{ inputs.needs_android }}" ;;
-          web)       NEEDS="${{ inputs.needs_web }}" ;;
+          ios|macos) NEEDS="$NEEDS_IOS" ;;
+          android)   NEEDS="$NEEDS_ANDROID" ;;
+          web)       NEEDS="$NEEDS_WEB" ;;
           *)
             echo "::warning::Unknown platform '$PLATFORM' — not skipping."
             echo "skip=false" >> "$GITHUB_OUTPUT"