Move GOLANG_FIPS=1 back into tests:fips CI job

Enabling GOLANG_FIPS=1 (from !1326 (merged)) as part of the build process caused a build failure in https://gitlab.com/gitlab-org/omnibus-gitlab/-/jobs/11294885432:

    panic: opensslcrypto: can't enable FIPS mode for OpenSSL 1.1.1f  31 Mar 2020: FIPS_mode_set
	openssl error(s):
	error:0F06D065:common libcrypto routines:FIPS_mode_set:fips mode not supported
		../crypto/o_fips.c:22
goroutine 1 [running]:
crypto/internal/backend.init.0()
	crypto/internal/backend/openssl.go:77 +0x254
make: *** [Makefile:116: bin/gitlab-shell] Error 2