Update gpg incantations to work with 2.4.1

There's some weird new behavior with keyring paths and keyboxd,
so avoid the entire footgun by just explicitly telling it what key to use.

firefox-overlay.nix

@@ -96,10 +96,9 @@ let
       FILE = chksum;
       ASC = chksumSig;
     } ''
-      HOME=`mktemp -d`
       set -eu
-      gpg --import < ${pgpKey}
-      gpgv --keyring=$HOME/.gnupg/pubring.kbx $ASC $FILE
+      gpg --dearmor < ${pgpKey} > keyring.gpg
+      gpgv --keyring=./keyring.gpg $ASC $FILE
       mkdir $out
     '';
 
@@ -131,11 +130,10 @@ let
         postFetch =
           let asc = super.fetchurl { url = info.sig; sha512 = info.sigSha512; }; in ''
           : # Authenticity Check
-          HOME=`mktemp -d`
           set -eu
           export PATH="$PATH:${self.gnupg}/bin/"
-          gpg --import < ${pgpKey}
-          gpgv --keyring=$HOME/.gnupg/pubring.kbx ${asc} $out
+          gpg --dearmor < ${pgpKey} > keyring.gpg
+          gpgv --keyring=./keyring.gpg ${asc} $out
         '';
       };