Fix GitHub Actions workflow: secrets context not allowed in if conditions

Move secrets to job-level env vars and use env context in the conditional
expression to check whether Sonatype credentials are available before
publishing snapshot artifacts.

Author: thibauult

.github/workflows/build.yml

@@ -12,6 +12,10 @@ jobs:
 
     runs-on: ubuntu-latest
 
+    env:
+      MAVEN_USERNAME: ${{ secrets.MAVEN_USERNAME }}
+      MAVEN_PASSWORD: ${{ secrets.MAVEN_PASSWORD }}
+
     steps:
     - uses: actions/checkout@v4
 
@@ -27,7 +31,7 @@ jobs:
         arguments: build jacocoTestReport jacocoTestCoverageVerification
 
     - name: Publish SNAPSHOT artifacts
-      if: github.event_name == 'pull_request' && secrets.MAVEN_USERNAME != '' && secrets.MAVEN_PASSWORD != ''
+      if: github.event_name == 'pull_request' && env.MAVEN_USERNAME != ''
       uses: gradle/gradle-build-action@v2.4.2
       with:
         arguments: publishToSonatype -PmavenRepoUsername=${{ secrets.MAVEN_USERNAME }} -PmavenRepoPassword=${{ secrets.MAVEN_PASSWORD }}