Simplify make.sh to use a single target instead of targets

[prasannavl]

/kind refactor

• Change TARGETS env var into TARGET. This brings consistency to how all the commands behave. Previously some commands looped over all, while some didn't.
• Multi-targets can easily be involved by just calling the commands with different targets set (an use-case that's not seen a necessity so far, other than local, where it's easy to run separately)

Additionally:

• This also uses major version numbering instead of hashes for GH Actions, which make it easier to maintain and comprehend, while also taking advantage of semver based fixes. Assumption: Rely on GitHub official actions to use major versions safely. The benefits outweigh the reproduciblity guarantee of hashes, when dealing with well known and relied on GH actions.
• This also makes an attempt to fix the detached HEAD checkouts, that result in -dirty tags being added to release builds.