Fix `check-vendored-charts.sh` to actually work

What does this MR do?

Part of gitlab-com/gl-infra/delivery#2147 (closed)

The script in question had flawed logic where it was pushd into the temp directory and running from there, meaning it was diffing everything against the same copy (not the copy in this repo). This mean that the script would always succeed, and thus not be useful.

This change fixes it to work correctly (and fixes an issue found when testing the vendored chart for gitlab-runner).

Author Check-list

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Reviewer Check-list

• Reviewed the diff jobs to confirm changes are as expected
• No changes shown in the diffs not associated with this MR - This may require a rebase or further investigation

Applier Check-list

• Make sure there is no ongoing deployment for the affected envs before merging (see #announcements slack channel)