Fix gitaly install on systems where env doesn't support the -u flag

What does this MR do?

On some systems, i.e. I ran into trouble on OpenBSD, /usr/bin/env doesn't support the -u flag. However, it's unconditionally used in lib/tasks/gitlab/gitaly.rb.

So, before using env with -u flags, first test whether env supports them, and only make use of the -u flag, when env supports it.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

I tried hard to add some tests covering the cases where env supports or not supports the -u flag, however, I wasn't able to get things right.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team