github: Update the environment used for doing msys2 builds

This should get a patched host compiler for the clang64 environment,
supporting symbol visibility, avoiding exporting too many symbols
from the dylibs.

For the mingw64 build (with GCC, which doesn't support symbol
visibility on Windows), the number of exported symbols
seems to be slightly lower, so we don't hit the limit of exported
symbols quite yet.

.github/workflows/build.yml

@@ -158,6 +158,11 @@ 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