feat: Better lint and test for devs and Makefile cleanup.

[smoser]

This change does several things all towards the end of making
it easier for developers to run tests and lint on go code.

Now a developer can run either 'make go-test' or 'make lint'
without having lots of C dependencies or running in a specific
container or stacker.

Things changed:

• Add a build tag 'skipembed' that builds without needing
  cmd/stacker/lxc-wrapper/lxc-wrapper . This allows you to
  build (or test) without that file. In order to ensure that
  you do not get very far trying to run a 'stacker' binary,
  I've added a 'panic' if stacker binary is built with skipembed.

• Add download-tools target to makefile to ease in downloading of
  regctl and zot.

• Move the downloading of golangci-lint from the github workflow
  to Makefile. This makes it easier for developer to get it.
  It is also added to the 'download-tools' target.

• be specific about the version of golangci-lint that is used.
  1.54.2. golangci-lint's docs specifically say to do this:

  IMPORTANT: It's highly recommended installing a specific version
  of golangci-lint available on the releases page.

  changing it is simply a matter of changing the value in the makefile.

• Move running of 'go test' out of the lint target to its own
  'go-test' target. go-test target will now write a coverage.html
  so you can view coverage easily.

• add 'dlbin' makefile "function" for downloading binaries, and use
  that for regctl and zot from their rules.

[codecov]

Codecov Report

Merging #511 (af09c66) into main (3eb5cf6) will increase coverage by 11.89%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           main     #511       +/-   ##
=========================================
+ Coverage      0   11.89%   +11.89%     
=========================================
  Files         0       40       +40     
  Lines         0     5733     +5733     
=========================================
+ Hits          0      682      +682     
- Misses        0     4929     +4929     
- Partials      0      122      +122     

see 40 files with indirect coverage changes

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[smoser]

I had to re-push to adjust a commit message comment. so af09c66 is identical to the version @rchamarthy reviewed ( 934bce6 to af09c66) but has a different git message.

the difference in the commit message was just to drop the mention of moving coverage.txt as @rchincha asked that it not be moved.