Update CI workflow branch conditions

Replaced environment variable conditions with explicit branch checks for job execution in the CI workflow. This ensures jobs run only on 'main' or release branches, improving reliability and clarity.

Author: Sergio0694

.github/workflows/ci-build.yml

@@ -91,10 +91,12 @@ jobs:
         path: |
           ./*.nupkg
 
-  # Sign the packages for release
+  # Sign the packages for release.
+  # Note: here and below we're explicitly repeating the conditions, rather than using 'env.'.
+  # Doing so doesn't seem to work for conditions of jobs themselves, only for individual steps.
   sign:
     needs: [build-and-test]
-    if: ${{ env.IS_MAIN == 'true' || env.IS_RELEASE == 'true' }}
+    if: ${{ github.ref == 'refs/heads/main' || startsWith(github.ref, 'refs/heads/rel/') }}
     runs-on: windows-latest
     permissions:
       id-token: write # Required for requesting the JWT
@@ -155,7 +157,7 @@ jobs:
 
   # Push official packages to NuGet
   release:
-    if: ${{ env.IS_RELEASE == 'true' }}
+    if: ${{ startsWith(github.ref, 'refs/heads/rel/') }}
     needs: [sign]
     environment: nuget-release-gate # This gates this job until manually approved
     runs-on: ubuntu-latest