CI: Fix Meson mixup between Conda and system Python headers (#839)

* use Conda Meson instead of system Meson

Otherwise, Meson insists on getting Cython to include system Python
headers which simply doesn't work:

	-----
	Sanity check compile stderr:
	In file included from /usr/include/python3.10/Python.h:8,
	                 from sanity_check_for_cython.c:19:
	/usr/include/python3.10/pyconfig.h:9:12: fatal error: aarch64-linux-gnu/python3.10/pyconfig.h: No such file or directory
	    9 | #  include <aarch64-linux-gnu/python3.10/pyconfig.h>
	      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	compilation terminated.

	-----

Also, avoid starting a new shell!

* Create the `test` env

* Different strategy: get rid of Conda altogther

Conda was used to get GCC or Clang for compiling the C extensions.
GitHub Actions runners come with C/C++ compilers pre-installed.

Author: DimitriPapadopoulos

.github/workflows/ci.yaml

@@ -27,25 +27,11 @@ jobs:
           submodules: recursive
           fetch-depth: 0 # required for version resolution
 
-      - name: Set up Conda
-        uses: conda-incubator/setup-miniconda@v3.2.0
+      - name: Set up Python
+        uses: actions/setup-python@v6
         with:
-          channels: conda-forge
-          miniforge-version: latest
           python-version: ${{ matrix.python-version }}
-
-      - name: Install compilers
-        run: conda install -y c-compiler cxx-compiler
-
-      - name: Install clang
-        if: matrix.platform == 'macos-15-large'
-        run: conda install -y 'clang>=12.0.1,<17'
-
-      - name: Show conda environment info
-        run: |
-          conda info
-          conda config --show-sources
-          conda list --show-channel-urls
+          cache: 'pip'
 
       - name: Install numcodecs
         run: |
@@ -93,15 +79,11 @@ jobs:
           submodules: recursive
           fetch-depth: 0
 
-      - name: Set up Conda
-        uses: conda-incubator/setup-miniconda@v3.2.0
+      - name: Set up Python
+        uses: actions/setup-python@v6
         with:
-          channels: conda-forge
-          miniforge-version: latest
-          python-version: "3.13"
-
-      - name: Install compilers
-        run: conda install -y c-compiler cxx-compiler
+          python-version: ${{ matrix.python-version }}
+          cache: 'pip'
 
       - name: Install numcodecs
         run: |
@@ -147,15 +129,11 @@ jobs:
           submodules: recursive
           fetch-depth: 0 # required for version resolution
 
-      - name: Set up Conda
-        uses: conda-incubator/setup-miniconda@v3.2.0
+      - name: Set up Python
+        uses: actions/setup-python@v6
         with:
-          channels: conda-forge
-          miniforge-version: latest
-          python-version: "3.13"
-
-      - name: Install compilers
-        run: conda install -y c-compiler cxx-compiler
+          python-version: ${{ matrix.python-version }}
+          cache: 'pip'
 
       - name: Install numcodecs and Zarr
         run: |