support gccEmacs

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

nix-integration.patch

@@ -26,3 +26,12 @@ index c8cfb0495..d8ed107bd 100644
    "Directory for volatile local storage.
  
  Use this for files that change often, like cache files. Must end with a slash.")
+@@ -276,7 +276,7 @@ config.el instead."
+ ;; Don't store eln files in ~/.emacs.d/eln-cache (they are likely to be purged
+ ;; when upgrading Doom).
+ (when (boundp 'comp-eln-load-path)
+-  (add-to-list 'comp-eln-load-path (concat doom-cache-dir "eln/")))
++  (add-to-list 'comp-eln-load-path (concat doom-local-dir "cache/eln/")))
+ 
+ (after! comp
+   ;; HACK `comp-eln-load-path' isn't fully respected yet, because native