src/utils/pathUtils: return the default go path only if exists

https://go-review.googlesource.com/c/vscode-go/+/276493 meant
to add /usr/local/bin/go to the default path to search, but introduced
a bug - we shouldn't return the path if the go binary doesn't exist.

TODO: This function got complicated over time and deserves proper testing.

For #971
Fixes #1065

Change-Id: I3ddf90983a1f66d6d76a3f9a50eb528a93904f3c
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/279727
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

src/utils/pathUtils.ts

@@ -104,8 +104,8 @@ export function getBinPathWithPreferredGopathGorootWithExplanation(
 		for (const p of defaultPathsForGo) {
 			if (executableFileExists(p)) {
 				binPathCache[toolName] = p;
+				return {binPath: p, why: 'default'};
 			}
-			return {binPath: p, why: 'default'};
 		}
 		return {binPath: ''};
 	}