Support signing Flatpak builds with GPG

This honors the following environment variables:

- $GPG_KEY_ID: When set, it can be the identifier, full fingerprint, or
  email address of a GnuPG key which will be used to sign Flatpak builds.

- $GPG_HOMEDIR: Path to an alternative home directory to be used by GnuPG,
  instead of the default ~/.gnupg directory of the current user. This can
  be used to point GnuPG to a different keychain.

build/gulpfile.vscode.linux.js

@@ -217,6 +217,13 @@ function buildFlatpak(arch) {
 		arch: flatpakArch,
 		bundlePath: manifest.appId + '-' + flatpakArch + '.flatpak',
 	};
+	// Setup PGP signing if requested.
+	if (process.env.GPG_KEY_ID !== undefined) {
+		buildOptions.gpgSign = process.env.GPG_KEY_ID;
+		if (process.env.GPG_HOMEDIR) {
+			buildOptions.gpgHomedir = process.env.GPG_HOME_DIR;
+		}
+	}
 	return function (cb) {
 		require('flatpak-bundler').bundle(manifest, buildOptions, cb);
 	}