Makefile: Make CGO_ENABLED configurable

[chewi]

What this PR does / why we need it:

Although disabling CGO_ENABLED can have some benefits, it is sometimes needed for compliance reasons.

Please check the following list:

• [ ] Does the affected code have corresponding tests, e.g. unit test, E2E test? -- N/A
• [ ] Does this change require a documentation update? -- No
• [ ] Does this introduce breaking changes that would require an announcement or bumping the major version? -- No
• [ ] Do all new files have an appropriate license header? -- N/A