fix: reverse quotes for variable expansion

Without reversing quotes, the variables are sometimes improperly expanded, resulting in files not being removed.

plugins/certs/subcommands/generate

@@ -15,7 +15,7 @@ certs_generate_cmd() {
   if [[ ! -f "$APP_SSL_PATH/server.key" ]] && [[ ! -f "$APP_SSL_PATH/server.crt" ]]; then
     local CERTS_GENERATE_TMP_WORK_DIR=$(mktemp -d "/tmp/dokku_certs.XXXXXXXXX")
     pushd "$CERTS_GENERATE_TMP_WORK_DIR" >/dev/null
-    trap 'popd &>/dev/null || true; rm -rf "$CERTS_GENERATE_TMP_WORK_DIR" >/dev/null' INT TERM EXIT
+    trap "popd &>/dev/null || true; rm -rf '$CERTS_GENERATE_TMP_WORK_DIR' >/dev/null" INT TERM EXIT
 
     openssl genrsa -out server.key 2048
     openssl req -new -key server.key -out server.csr