hooks: Fix shebang on pre-commit hook

The pre-commit hook uses features that are not defined in the POSIX
shell. Fix the shebang so that it is correctly identified as a bash
script.

Part-of: <https://gitlab.gnome.org/GNOME/fractal/-/merge_requests/1111>

hooks/pre-commit.hook

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 # Depends on: scripts/checks.sh
 
 # Style helpers