Revert "github: Update the environment used for doing msys2 builds"

This reverts commit 55db22d242.

The base tarball for setup-msys2 should now be new enough to
include the patched compiler in the clang64 environment, so this
no longer requires updating the environment on each build.

.github/workflows/build.yml

@@ -165,11 +165,6 @@ jobs:
       - uses: msys2/setup-msys2@v2
         with:
           msystem: ${{matrix.sys}}
-          # Update so that we get a new enough compiler in the clang64
-          # environment (a version of clang that supports hidden visibility).
-          # The GCC build (in the mingw64 environment) seems to export
-          # slightly fewer symbols and doesn't hit the limit quite yet.
-          update: true
           install: >-
             git
             make