[static-annex] Statically linked git-annex builds

[Kyle Meyer <kyle@kyleam.com>]

On some machines, I want to install git-annex under a user's home
directory without touching the wider system.  These machines don't have
the tools for building git-annex from source (and, even if they did, I'd
rather not wait for the build).

Two possible solutions would be to

 1) install it from conda-forge

    https://git-annex.branchable.com/install/conda/

 2) download the git-annex's standalone build

    https://git-annex.branchable.com/install/Linux_standalone/
    https://joeyh.name/blog/entry/completely_linux_distribution-independent_packaging/

Both of these are nice options to have.  I'm unlikely to jump at *1*
given I'm not a Conda user.  That leaves *2*.

The standalone build has been around for a long time and has surely seen
a good amount of use (including via NeuroDebian's backports).  If I
recall correctly, there's some noticeable overhead.  And, for my use
case, I don't need external programs like Git and Rsync to be bundled.
But it would very likely do.

Still, I wondered how much effort it'd be to create a statically linked
build.  Searching around, I found that Pandoc uses the benz0li/ghc-musl
image for this purpose.

  https://github.com/jgm/pandoc/blob/4c66bf27903ca50fccfb0b025a9d5cde217a6cec/Makefile#L164-L176
  https://gitlab.com/benz0li/ghc-musl

With that very helpful starting point, I've put together the scripts at
<https://git.kyleam.com/static-annex/> to create statically linked
git-annex builds.  (For my current use cases, I'm concerned with only
x86_64 GNU/Linux machines, and I've disabled the webapp and assistant
features, as I don't use them.)

Everything seems to be working as expected.  For now, I'll continue to
use the latest version of these builds as my primary git-annex binary to
hopefully catch any issues.

I've uploaded the three most recent git-annex releases to
<https://dl.kyleam.com/git-annex/>.