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.

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
 -------
 

archiso/mkarchiso

@@ -335,8 +335,8 @@ _make_custom_airootfs() {
 _make_packages() {
     _msg_info "Installing packages to '${pacstrap_dir}/'..."
 
-    if [[ -n "${gpg_key}" ]]; then
-        exec {ARCHISO_GNUPG_FD}<>"${work_dir}/pubkey.gpg"
+    if [[ -v gpg_publickey ]]; then
+        exec {ARCHISO_GNUPG_FD}<>"$gpg_publickey"
         export ARCHISO_GNUPG_FD
     fi
     if [[ -v cert_list[0] ]]; then
@@ -364,7 +364,7 @@ _make_packages() {
         exec {ARCHISO_TLSCA_FD}<&-
         unset ARCHISO_TLSCA_FD
     fi
-    if [[ -n "${gpg_key}" ]]; then
+    if [[ -v gpg_publickey ]]; then
         exec {ARCHISO_GNUPG_FD}<&-
         unset ARCHISO_GNUPG_FD
     fi
@@ -1614,8 +1614,10 @@ _set_overrides() {
 }
 
 _export_gpg_publickey() {
-    rm -f -- "${work_dir}/pubkey.gpg"
-    gpg --batch --no-armor --output "${work_dir}/pubkey.gpg" --export "${gpg_key}"
+    gpg_publickey="${work_dir}/pubkey.gpg"
+    rm -f -- "$gpg_publickey"
+    gpg --batch --no-armor --output "$gpg_publickey" --export "${gpg_key}"
+    [[ -s "$gpg_publickey" ]] || return
 }
 
 _make_version() {