RBE is only supported when the host is linux, so don't try on others.

Bug: 343589123
Test: m and verify
Change-Id: I872d36a40c980f58c5de2598c3fb5dcfcadabf4f

ui/build/config.go

@@ -1251,6 +1251,11 @@ func (c *configImpl) StartGoma() bool {
 }
 
 func (c *configImpl) canSupportRBE() bool {
+	// Only supported on linux
+	if runtime.GOOS != "linux" {
+		return false
+	}
+
 	// Do not use RBE with prod credentials in scenarios when stubby doesn't exist, since
 	// its unlikely that we will be able to obtain necessary creds without stubby.
 	authType, _ := c.rbeAuth()