ci: fix installation of meson

Author: apprehensions

.github/workflows/main.yml

@@ -46,7 +46,7 @@ jobs:
     - uses: actions/setup-python@v1
       with:
         python-version: '3.x'
-    - run: pip install meson ninja
+    - run: pip3 install meson ninja
 
     # Fetch tags to determine proper version number inside git
     - name: fetch tags