alacritty-theme: fix install by packaging *.toml files

Without the change `alacritty-tmeme` fails the build in `master` as:

    $ nix build --no-link -f. -L alacritty-theme
    Running phase: unpackPhase
    unpacking source archive /nix/store/fspc2v4jrpfx30avp5aagfigq0vhy8pz-source
    source root is source/themes
    Running phase: patchPhase
    Running phase: updateAutotoolsGnuConfigScriptsPhase
    Running phase: installPhase
    install: missing file operand
    Try 'install --help' for more information.

pkgs/data/themes/alacritty-theme/default.nix

@@ -22,7 +22,7 @@ stdenvNoCC.mkDerivation (self: {
   sourceRoot = "${self.src.name}/themes";
   installPhase = ''
     runHook preInstall
-    install -Dt $out *.yaml
+    install -Dt $out *.toml
     runHook postInstall
   '';