move FIPS related env vars inline for FIPS unit tests (#10960)

* move FIPS related env vars inline for FIPS unit tests

* Add inline comment

* Only keep GODEBUG=fips140=only in the command

* Clarify comment

---------

Co-authored-by: Shaunak Kashyap <[email protected]>

Author: pchila

.buildkite/pipeline.yml

@@ -66,10 +66,12 @@ steps:
 
       - label: "Unit tests - fips140=only Ubuntu 22.04"
         key: "unit-tests-2204-fips140-only"
-        command: ".buildkite/scripts/steps/unit-tests.sh"
+        # Note: The GODEBUG=fips140=only environment variable must be set in the command itself (as opposed to
+        # in the env block) so that it is applied *only* to the 'go' command invoked by the script, and
+        # not to any other Go code executed as part of the Buildkite agent itself.
+        command: 'GODEBUG="fips140=only" .buildkite/scripts/steps/unit-tests.sh'
         env:
           FIPS: "true"
-          GODEBUG: "fips140=only"
         artifact_paths:
           - "build/TEST-*.html"
           - "build/TEST-*.xml"