Ensure Workhorse is built in FIPS mode

What does this MR do?

Previously Workhorse was not compiled properly for FIPS since the fips build tag was not defined, so LabKit's fips.Enabled() would erroneously return false. The FIPS_MODE=1 environment variable makes Workhorse activate the right build settings.

This came out an investigation of Maven package handling in gitlab#380559 (closed).

Related issues

gitlab#380559 (closed)

Testing

[stanhu@stanhu-fips1 bin]$ ./gitlab-workhorse
INFO[0000] Starting                                      build_time=20221216.094924 version=11-10-0cfa69752d8-74ffd66ae-ee-187173-g031da1d300c1
INFO[0000] FIPS mode is enabled. Using an external SSL library.
INFO[0000] Running upstream server                       address="localhost:8181" network=tcp

Checklist

See Definition of done.

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Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
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Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
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