mkarchiso: do not try to use an non existent GPG public key file

The `bootstrap` build mode never calls `_export_gpg_publickey`, so even if
the GPG key is passed with the `-g` option and thus the `gpg_key` variable
is set, the `${work_dir}/pubkey.gpg` file will not exist.
This has not caused any issue so far because the `ARCHISO_GNUPG_FD` file
descriptor opens the file for both reading and writing, which means the
file gets created if it does not exist.

Assign the exported public key file name to a `gpg_publickey` variable in
`_export_gpg_publickey` and check for it when the file is used.

Since the exist status of the gpg command cannot be checked, look for the
exported public key file instead.

1 files changed, 2 insertions, 0 deletions

diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 5115cb1..982c722 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -12,6 +12,8 @@ Added
 Changed
 -------
 
+- Check if the GPG public key file was successfully placed in the work directory before trying to use it.
+
 Removed
 -------