fix(build): "package" task fails on feature-branches #3818

Problem:
The "Package Test" CI job fails on `feature/x` branches that set `betaUrl`:

    Error: Cannot package VSIX as both a release and a beta simultaneously
        at main (/codebuild/output/src3161342786/src/github.com/aws/aws-toolkit-vscode-staging/scripts/build/package.ts:111:19)

(Note: `feature/x` branches are auto-tagged as `pre-x`, e.g. https://github.com/aws/aws-toolkit-vscode/releases/tag/pre-ec2 )

Solution:
Make isRelease() more precise.

Author: justinmk3

scripts/build/package.ts

@@ -68,11 +68,12 @@ function parseArgs() {
 }
 
 /**
- * If the current commit is tagged then it is a "release build", else it is
- * a prerelease/nightly/edge/preview build.
+ * If the _current_ commit is tagged as a release ("v1.26.0") then it is a "release build", else it
+ * is a prerelease/nightly/edge/preview build.
  */
 function isRelease(): boolean {
-    return child_process.execSync('git tag -l --contains HEAD').toString() !== ''
+    const tag = child_process.execSync('git tag -l --contains HEAD').toString()
+    return !!tag?.match(/v\d+\.\d+\.\d+/)
 }
 
 /**