Deprecate --install-grub in favor of --install-bootloader for nixos-rebuild.

Fixes #14293

nixos/doc/manual/man-nixos-rebuild.xml

···
     </group>
     <sbr />
     <arg><option>--upgrade</option></arg>
-    <arg><option>--install-grub</option></arg>
+    <arg><option>--install-bootloader</option></arg>
     <arg><option>--no-build-nix</option></arg>
     <arg><option>--fast</option></arg>
     <arg><option>--rollback</option></arg>
···
   </varlistentry>
 
   <varlistentry>
-    <term><option>--install-grub</option></term>
+    <term><option>--install-bootloader</option></term>
     <listitem>
-      <para>Causes the GRUB boot loader to be (re)installed on the
-      device specified by the
-      <varname>boot.loader.grub.device</varname> configuration
-      option.</para>
+      <para>Causes the boot loader to be (re)installed on the
+      device specified by the relevant configuration options.
+      </para>
     </listitem>
   </varlistentry>
 

nixos/modules/installer/tools/nixos-install.sh

···
 # configuration.
 echo "finalising the installation..."
 if [ -z "$noBootLoader" ]; then
-  NIXOS_INSTALL_GRUB=1 chroot $mountPoint \
+  NIXOS_INSTALL_BOOTLOADER=1 chroot $mountPoint \
       /nix/var/nix/profiles/system/bin/switch-to-configuration boot
 fi
 

nixos/modules/installer/tools/nixos-rebuild.sh

···
         action="$i"
         ;;
       --install-grub)
-        export NIXOS_INSTALL_GRUB=1
+        echo "$0: --install-grub deprecated, use --install-bootloader instead" >&2
+        export NIXOS_INSTALL_BOOTLOADER=1
+        ;;
+      --install-bootloader)
+        export NIXOS_INSTALL_BOOTLOADER=1
         ;;
       --no-build-nix)
         buildNix=

nixos/modules/system/boot/loader/grub/install-grub.pl

···
 my $versionDiffer = get("fullVersion") ne $prevGrubState->version;
 my $efiDiffer = $efiTarget ne $prevGrubState->efi;
 my $efiMountPointDiffer = $efiSysMountPoint ne $prevGrubState->efiMountPoint;
-my $requireNewInstall = $devicesDiffer || $nameDiffer || $versionDiffer || $efiDiffer || $efiMountPointDiffer || (($ENV{'NIXOS_INSTALL_GRUB'} // "") eq "1");
+if (($ENV{'NIXOS_INSTALL_GRUB'} // "") eq "1") {
+    warn "NIXOS_INSTALL_GRUB env var deprecated, use NIXOS_INSTALL_BOOTLOADER";
+    $ENV{'NIXOS_INSTALL_BOOTLOADER'} = "1";
+}
+my $requireNewInstall = $devicesDiffer || $nameDiffer || $versionDiffer || $efiDiffer || $efiMountPointDiffer || (($ENV{'NIXOS_INSTALL_BOOTLOADER'} // "") eq "1");
 
 # install a symlink so that grub can detect the boot drive
 my $tmpDir = File::Temp::tempdir(CLEANUP => 1) or die "Failed to create temporary space";

nixos/modules/system/boot/loader/systemd-boot/systemd-boot-builder.py

···
 import glob
 import tempfile
 import errno
+import warnings
 
 def copy_if_not_exists(source, dest):
     if not os.path.exists(dest):
···
 parser.add_argument('default_config', metavar='DEFAULT-CONFIG', help='The default NixOS config to boot')
 args = parser.parse_args()
 
-# We deserve our own env var!
 if os.getenv("NIXOS_INSTALL_GRUB") == "1":
+    warnings.warn("NIXOS_INSTALL_GRUB env var deprecated, use NIXOS_INSTALL_BOOTLOADER", DeprecationWarning)
+    os.environ["NIXOS_INSTALL_BOOTLOADER"] = "1"
+
+if os.getenv("NIXOS_INSTALL_BOOTLOADER") == "1":
     if "@canTouchEfiVariables@" == "1":
         subprocess.check_call(["@systemd@/bin/bootctl", "--path=@efiSysMountPoint@", "install"])
     else: