Fall back to GitHub-provided environment variable when running in GHA

Author: grynspan

Package.swift

@@ -479,6 +479,10 @@ extension Array where Element == PackageDescription.CXXSetting {
       result.append(.define("SWT_TESTING_LIBRARY_COMMIT_HASH", to: #""\#(git.currentCommit)""#))
       if git.hasUncommittedChanges {
         result.append(.define("SWT_TESTING_LIBRARY_COMMIT_MODIFIED", to: "1"))
+      } else if let gitHubSHA = Context.environment["GITHUB_SHA"] {
+        // When building in GitHub Actions, the git command may fail to get us the
+        // commit hash, so check if GitHub shared it with us instead.
+        result.append(.define("SWT_TESTING_LIBRARY_COMMIT_HASH", to: #""\#(gitHubSHA)""#))
       }
     }