Only run source gitpod log file if it exists

What does this MR do and why?

This PR adds a check to see if gitpod log file exists, and if so, run source /workspace/gitpod_start_time.sh command in Gitpod workspace.

Closes gitlab-development-kit#1538 (closed)

Screenshots or screen recordings

Tested this MR's change by checking non-existing file here and ensured gdk commands still succeed to run and Gitlab URL is ready to open.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.