systemd-boot: Unlink loader.conf if it exists

Since systemd version 232 the install subcommand of bootctl opens the
loader.conf with fopen() modes "wxe", where the "e" stands for
exclusive, so the call will fail if the file exists.

For installing the boot loader just once this is fine, but if we're
using NIXOS_INSTALL_BOOTLOADER on a systemd where the bootloader is
already present this will fail.

Exactly this is done within the simpleUefiGummiboot installer test,
where nixos-install is called twice and thus the bootloader is also
installed twice, resulting in an error during the fopen call:

Failed to open loader.conf for writing: File exists

Removing the file prior to calling bootctl should fix this.

I've tested this using the installer.simpleUefiGummiboot test and it now
succeeds.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @edolstra, @shlevy, @mic92
Fixes: #22925