CI: rebase MSYS2 dll

This fixes, or should make it less likely, spurious CI failures because
of:
`child_info_fork::abort: address space needed by <DLL> is already occupied`

The issue is that Unix `fork()`, given how it works, preserves
libraries' addresses. Windows does not have such a function, so Cygwin
needs emulate it by moving the libraries in a child process to match
the addresses in its parent. This leads to conflicts if Windows already
loaded something there.

As a workaround, Cygwin has a `rebase` application to assign specific
addresses to each DLL and forcing Windows to use those. This generally
fixes the issue (until a DLL is updated that is, but that's not
a concern for CI since everything is rebuilt from scratch every time).

In the case of #12515 though, the failing DLL is a temporary one built
during the compilation. So a rebase of MSYS2 packages will not quite
fix the problem. However, by moving other DLLs at specific locations,
it reduce the risk of collision to only be between the temporary ones.

Fixes #12515

Closes #12521

.github/workflows/test.yml

@@ -164,10 +164,17 @@ jobs:
       with:
         update: true
         msystem: MSYS
+      id: msys2
     - name: Install deps
       # Not using setup-msys2 `install` option to make it easier to copy/paste
       run: |
         pacman --noconfirm -S --needed git rust
+    - name: rebase
+      env:
+        MSYS2_LOCATION: ${{ steps.msys2.outputs.msys2-location }}
+      shell: cmd
+      run: |
+        "%MSYS2_LOCATION%\usr\bin\dash" /usr/bin/rebaseall -p -v
     - name: cargo build
       run: |
         cargo build