CI: fix installation of specific meson version in msvc job

The error was not spotted before because the job is not run with a
meson version specified in the job configuration matrix, thus this
part of the setup was not exercised.

Author: dnicolodi

.github/workflows/tests.yml

@@ -176,7 +176,7 @@ jobs:
           architecture: x64
 
       - name: Install Meson
-        run: python -m pip install "meson==${{ matrix.meson }}"
+        run: python -m pip install "meson ${{ matrix.meson }}"
         if: ${{ matrix.meson }}
 
       - name: Install