treewide: Fix mysql alias deprecation breakage

62733b37b4 broke evaluation in all
places `pkgs.mysql` was used. Fix this by changing all occurrences to
`pkgs.mariadb`.

(cherry picked from commit 59e0120aa5c1241d48048afa615e25c65d7e366d)

nixos/modules/services/backup/mysql-backup.nix

@@ -4,7 +4,7 @@ with lib;
 
 let
 
-  inherit (pkgs) mysql gzip;
+  inherit (pkgs) mariadb gzip;
 
   cfg = config.services.mysqlBackup;
   defaultUser = "mysqlbackup";
@@ -20,7 +20,7 @@ let
   '';
   backupDatabaseScript = db: ''
     dest="${cfg.location}/${db}.gz"
-    if ${mysql}/bin/mysqldump ${if cfg.singleTransaction then "--single-transaction" else ""} ${db} | ${gzip}/bin/gzip -c > $dest.tmp; then
+    if ${mariadb}/bin/mysqldump ${if cfg.singleTransaction then "--single-transaction" else ""} ${db} | ${gzip}/bin/gzip -c > $dest.tmp; then
       mv $dest.tmp $dest
       echo "Backed up to $dest"
     else