Restore workflow file as existed prior to PR #635 (#647)

### Motivation and Context

This restores a change required in PR #635 -- comment out the freeze on
the yarn.lock file during install yarn in the workflow. This is because
the file that needed to be changed in the PR was the yarn.lock file
itself.

### Description

This removes the # that was addin in PR #635 to the workflow file.

---------

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Mollie Munoz <momuno@microsoft.com>

.github/workflows/copilot-build-frontend.yml

@@ -29,7 +29,7 @@ jobs:
           #!/usr/bin/env bash
           set -e # exit with nonzero exit code if anything fails
           echo "Running yarn install, yarn build, & yarn format"
-          yarn install #--frozen-lockfile # install dependencies and ensure lockfile is unchanged.
+          yarn install --frozen-lockfile # install dependencies and ensure lockfile is unchanged.
           yarn build # run build script
           yarn format # run format script
         working-directory: webapp