borgmatic: Do not inhibit idle in service (nix-community#3637)

This reflects a systemd service sample file change made in borgmatic
1.7.6, commit 2e9f70d49647d47fb4ca05f428c592b0e4319544:

    When backing up a machine with a monitor using logind to control
    idle timeout and things like DPMS, borgmatic can block the screen
    from turning on/off with systemd-inhibit. This is because by
    default systemd-inhibit will block
    "idle:sleep:shutdown". Borgmatic does not need to care about idle,
    only about suspend and shutdown. So, add an explicit `--what` flag
    for what borgmatic should inhibit.

    For more information see systemd-inhibit(1).

modules/services/borgmatic.nix

@@ -63,6 +63,7 @@ in {
           ExecStart = ''
             ${pkgs.systemd}/bin/systemd-inhibit \
               --who="borgmatic" \
+              --what="sleep:shutdown" \
               --why="Prevent interrupting scheduled backup" \
               ${programConfig.package}/bin/borgmatic \
                 --stats \

tests/modules/services/borgmatic/basic-configuration.service

@@ -2,6 +2,7 @@
 CPUSchedulingPolicy=batch
 ExecStart=/nix/store/00000000000000000000000000000000-systemd/bin/systemd-inhibit \
   --who="borgmatic" \
+  --what="sleep:shutdown" \
   --why="Prevent interrupting scheduled backup" \
   @borgmatic@/bin/borgmatic \
     --stats \