If built with gccEmacs, in `straight--native-compile-package` step tries
to access $HOME because `comp-eln-load-path` contains
"~/.emacs.d/eln-cache/". It errors on this as it is set to a readonly canary value of
`/homeless-shellter` in the nix build sandbox.
The workaround is twofold:
* Ignore `kill-emacs` from optimization. It would result in eln file
being written before doom sets up the load path outside of HOME.
* The above solution doesn't help in the actual build phase because the
worker processes compiling packages does not inherit this value but
it is set in them at a later time. This causes the build to fail
before `comp-eln-load-path` would be successfully overridden. Having
a writable $HOME does not seem to produce any build output there
hence the override and the check in `postInstall` to verify that the
directory is indeed empty.
The runtime value of `comp-eln-load-path` also had to be changed because
it is by default is located under the volatile `doom-cache-dir` which is
not a part of the derivation so the build time generated `.eln` files
couldn't be loaded from that.
References: #57
