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 <suzmue@golang.org>
Run-TryBot: Suzy Mueller <suzmue@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
(cherry picked from commit cd72252b1f88a434ccb4c7736cd13fb981eb2507)
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/340910
diff --git a/package.json b/package.json
index 118b6a2..a768597 100644
--- a/package.json
+++ b/package.json
@@ -464,7 +464,7 @@
               "type": "go",
               "request": "launch",
               "mode": "auto",
-              "program": "^\"\\${workspaceFolder}${1:}\""
+              "program": "^\"\\${fileDirname}${1:}\""
             }
           },
           {