kbd-util: allow to override the default keymap directories

This introduces $SYSTEMD_KEYMAP_DIRECTORIES environment variable to
override the hardcoded keymap directories.

I think it is not necessary to provide the first class configuration
option for controlling the keymap directories, but it is not good to
hardcode the paths. So, let's introduce an environment variable to
override that.

Prompted by #31759.

Closes #31759.

docs/ENVIRONMENT.md

@@ -348,12 +348,17 @@ All tools:
   `systemd-gpt-auto-generator` to ensure the root partition is mounted writable
   in accordance to the GPT partition flags.
 
-`systemd-firstboot` and `localectl`:
+`systemd-firstboot`, `localectl`, and `systemd-localed`:
 
 * `$SYSTEMD_LIST_NON_UTF8_LOCALES=1` — if set, non-UTF-8 locales are listed among
   the installed ones. By default non-UTF-8 locales are suppressed from the
   selection, since we are living in the 21st century.
 
+* `$SYSTEMD_KEYMAP_DIRECTORIES=` — takes a colon (`:`) separated list of keymap
+  directories. The directories must be absolute and normalized. If unset, the
+  default keymap directories (/usr/share/keymaps/, /usr/share/kbd/keymaps/, and
+  /usr/lib/kbd/keymaps/) will be used.
+
 `systemd-resolved`:
 
 * `$SYSTEMD_RESOLVED_SYNTHESIZE_HOSTNAME` — if set to "0", `systemd-resolved`