installer/tools/get-version-suffix: set --git-dir

The `nixos-rebuild` tool calls `get-version-suffix` to figure out the
git revision of the nixpkgs directory if there is a .git.

https://nvd.nist.gov/vuln/detail/CVE-2022-24765 made git throw an
error if the .git search logic is not turned off and a user
tries to access a `.git` directory they don’t own (otherwise a
different user could trick them into setting arbitrary git config).

So from now on we should always explicitely set `--git-dir`, which
turns this search logic (and thus the security check) off.

nixos/modules/installer/tools/get-version-suffix

···
 getVersion() {
     local dir="$1"
     rev=
-    if [ -e "$dir/.git" ]; then
+    gitDir="$dir/.git"
+    if [ -e "$gitDir" ]; then
         if [ -z "$(type -P git)" ]; then
             echo "warning: Git not found; cannot figure out revision of $dir" >&2
             return
         fi
         cd "$dir"
-        rev=$(git rev-parse --short HEAD)
-        if git describe --always --dirty | grep -q dirty; then
+        rev=$(git --git-dir="$gitDir" rev-parse --short HEAD)
+        if git --git-dir="$gitDir" describe --always --dirty | grep -q dirty; then
             rev+=M
         fi
     fi