package.json: use fileDirname for Launch Package snippet

Change the snippet to match the quick pick.

Change-Id: I4c2ab7ced9244b452b96d4d37e41e41ade79477d
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/340909
Trust: Suzy Mueller <[email protected]>
Run-TryBot: Suzy Mueller <[email protected]>
TryBot-Result: kokoro <[email protected]>
Reviewed-by: Hyang-Ah Hana Kim <[email protected]>
(cherry picked from commit cd72252)
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/340910

Author: suzmue

package.json

@@ -464,7 +464,7 @@
               "type": "go",
               "request": "launch",
               "mode": "auto",
-              "program": "^\"\\${workspaceFolder}${1:}\""
+              "program": "^\"\\${fileDirname}${1:}\""
             }
           },
           {