gopls/integration/govim: revert back to using the latest govim

A new version of govim with the fix for the original issue has been
released.

Change-Id: Iaea2eff7387f352cae1657949c5e364a66337f68
Reviewed-on: https://go-review.googlesource.com/c/tools/+/226557
Reviewed-by: Robert Findley <rfindley@google.com>
diff --git a/gopls/integration/govim/Dockerfile b/gopls/integration/govim/Dockerfile
index e4af733..f729e2f 100644
--- a/gopls/integration/govim/Dockerfile
+++ b/gopls/integration/govim/Dockerfile
@@ -17,4 +17,4 @@
 # a redundant copy of govim to the build cache using `go mod download`.
 RUN GOVIM_VERSION=$(go mod download -json github.com/govim/govim@latest | jq -r .Version) && \
     git clone https://github.com/govim/govim /src/govim && cd /src/govim && \
-    git checkout master
+    git checkout $GOVIM_VERSION