fix: add missing parentheses to core.isDebug()

[git-mracek]

This PR fixes a small but critical bug in src/main.ts that swallows the stderr output of failed commands when action debug logging is disabled.

The Issue:
Currently, around line 938 in the execSilent function, the code checks if debug logging is off:
if (!core.isDebug) {

Since core.isDebug is a function in the @actions/core package, !core.isDebug always evaluates to false. As a result, the fallback process.stderr.write(out.stderr); is never executed.

When a command (like tailscale up) fails, users only see a generic sudo failed with exit code X instead of the actual error message from the Tailscale CLI. This makes troubleshooting extremely difficult.

The Fix:
Added the missing parentheses (!core.isDebug()) so the function is actually called and evaluates the boolean correctly. Now, users will properly see the CLI errors in their action logs.

[mpminardi]

@git-mracek thanks for the fix, and apologies for the miss here!

[mpminardi]

FYI we'll need to do a make build and commit / push the results from that to resolve the linter error.

[git-mracek]

@mpminardi Done! I've run make build and pushed the updated dist/ files. The PR should be ready now.