devShell: guard against running from another directory

I was working on nix-eval-jobs with a dev shell with some shenanigans to
run against a locally built Lix and it was getting really annoying when
`nix develop ../lix#` was messing up my other git repo's hooks.

This is a fix via blunt force, but it is at least obvious how it works.

Change-Id: Ia29eeb5be57ab6a2c88451c00ea18a51e4dfe65e

.this-is-lix

@@ -0,0 +1 @@
+This is a file used by the dev shell shellHook in package.nix to check that this is actually a Lix repo before installing git hooks. Its contents have no meaning.

package.nix

@@ -476,7 +476,7 @@ stdenv.mkDerivation (finalAttrs: {
               # https://git.lix.systems/lix-project/lix/src/commit/7575db522e9008685c4009423398f6900a16bcce/src/nix/develop.cc#L240-L241
               # this is, of course, absurd.
               if [[ $name != lix-shell-env && $name != lix-shell-env-env ]]; then
-                return;
+                return
               fi
 
               PATH=$prefix/bin:$PATH
@@ -486,6 +486,11 @@ stdenv.mkDerivation (finalAttrs: {
               # Make bash completion work.
               XDG_DATA_DIRS+=:$out/share
 
+              if [[ ! -f ./.this-is-lix ]]; then
+                echo "Dev shell not started from inside a Lix repo, skipping repo setup" >&2
+                return
+              fi
+
               ${lib.optionalString (pre-commit-checks ? shellHook) pre-commit-checks.shellHook}
               # Allow `touch .nocontribmsg` to turn this notice off.
               if ! [[ -f .nocontribmsg ]]; then