CI: do not start upload if API key or passphrase is not set

CI periodically shows error like

  Failed to decrypt some environment variables

so it will be better to stop immediately when environment is inconsistent
for unknown reason (e.g. due to Github's problems).

scripts/build/ci/cirrus-ci_dispatcher.sh

@@ -118,6 +118,16 @@ if ! $DO_UPLOAD; then
 
 	./build-package.sh -a "$TERMUX_ARCH" -I $PACKAGE_NAMES
 else
+	if [ -z "$BINTRAY_API_KEY" ]; then
+		echo "[!] Can't upload without Bintray API key."
+		exit 1
+	fi
+
+	if [ -z "$BINTRAY_GPG_PASSPHRASE" ]; then
+		echo "[!] Can't upload without GPG passphrase."
+		exit 1
+	fi
+
 	# Workaround for concurrent uploads.
 	UPLOAD_DELAY=$((30 + RANDOM % 120))
 	echo "[!] Using workaround for Bintray issue with concurrent uploads."