src/goInstallTools: fix broken gocode-gomod install

?? is misused. It should be || because at some point
the extension sets GOBIN to ''.

Change-Id: I206e290da9a21abbfb1cc0a90a3fda6dec6a65d6
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/277293
Trust: Hyang-Ah Hana Kim <[email protected]>
Run-TryBot: Hyang-Ah Hana Kim <[email protected]>
TryBot-Result: kokoro <[email protected]>
Reviewed-by: Rebecca Stambler <[email protected]>

Author: hyangah

src/goInstallTools.ts

@@ -245,7 +245,7 @@ export async function installTool(
 		logVerbose(`install: %s %s\n%s%s`, goBinary, args.join(' '), stdout, stderr);
 
 		if (hasModSuffix(tool)) {  // Actual installation of the -gomod tool is done by running go build.
-			const gopath = env['GOBIN'] ?? env['GOPATH'];
+			const gopath = env['GOBIN'] || env['GOPATH'];
 			if (!gopath) {
 				return `GOBIN/GOPATH not configured in environment`;
 			}