home-environment: fix home.activation.checkLinkTargets

Problem
-------

We resolve symlinks from inside `/nix/store/HASH-home-manager-files`
into the nix store as `/nix/store/HASH-DRVNAME` which does not match
the pattern.

This happened to me because I pull in some repos in via `home.file`.
The `home-manager-files` derivation links to the repo's derivation in
the nix store. For example:

    let nanorcs = fetchFromGitHub {
      owner = "scopatz";
      repo = "nanorc";
      …
    }; in [
      {
        target = ".nano";
        source = nanorcs;
      }
      {
        target = ".nanorc";
        source = "${nanorcs}/nanorc";
      }
    ]

Solution
--------

Call `readlink` without `-e` to obtain only the first redirection from
`~` to `/nix/store/HASH-home-manager-files`.

modules/home-environment.nix

@@ -260,7 +260,7 @@ in
             relativePath="''${sourcePath#$newGenFiles/}"
             targetPath="$HOME/$relativePath"
             if [[ -e "$targetPath" \
-                && ! "$(readlink -e "$targetPath")" =~ "${pattern}" ]] ; then
+                && ! "$(readlink "$targetPath")" =~ "${pattern}" ]] ; then
               errorEcho "Existing file '$targetPath' is in the way"
               collision=1
             fi