ci: add VERSIONSTRING Makefile overwrite parameter

[shawnfunke]

The VERSIONSTRING parameter for the Makefile is required to be able
to skip the build step that runs git describe --always --dirty and
set the version suffix manually.

This change allows the packaging for Homebrew as during the build
process it cannot execute git commands.

This is a optional parameter and will fall back to the git command if
not set specifically.

make VERSIONSTRING=test DEST=buildir publishTo

Example usage of the parameter, would output a binary with the
version set to 1.0.0-test.

[artempyanykh]

Cool, thanks @shawnfunke! Do you plan to make a Homebrew formula for Marksman?

This change allows the packaging for Homebrew as during the build process it cannot execute git commands.

Quick question about this. What would the version string be like when building with Homebrew? Is it going to be hardcoded in the formula?

[shawnfunke]

@artempyanykh, yeah, it's supposed to be for homebrew/core (hopefully).

The version of the source code is based on the tag, for example:

  url "https://github.com/artempyanykh/marksman/archive/refs/tags/2022-12-19.tar.gz"
  sha256 "0627b47fa567d8f7655e71282f34807cefb4116c6d48f145683a29488f75cddf"

The version for the binary will probably contain the SHA, so for example
in the end it'd be like 1.0.0-<sha>, might not to be abbreviated but I don't
see that as a problem.

[artempyanykh]

Great! Thanks for the contribution @shawnfunke!

[shawnfunke]

Thanks for the fast merge! @artempyanykh would you mind to create a
new release so I can reference it in the formula, everything works
as expected.