Switch posix-cmake-test to use Ninja instead of Makefiles.

This is yet another workaround for hanging posix-cmake-test on Mac OS.
This enables parallel builds again, but uses Ninja build system instead
of Makefiles. Enabling parallel builds speeds up the build process and
releases Mac OS builders for other jobs sooner.

Additionally, fix incorrect apt invocations in CMake-related jobs. apt
is supposed to be used in interactive shells, and in scripts apt-get is
recommended. Also, apt-get update is needed before apt install, and
matrix.install must be transformed to a space-separated list. Also added
retries on network errors.

.github/workflows/ci.yml

@@ -621,7 +621,9 @@ jobs:
     steps:
       - name: Install packages
         if: matrix.install
-        run: sudo apt install ${{matrix.install}}
+        run: |
+          sudo apt-get -o Acquire::Retries=$NET_RETRY_COUNT update
+          sudo apt-get -o Acquire::Retries=$NET_RETRY_COUNT -y ${{join(matrix.install, ' ')}}
 
       - name: Setup Boost
         run: |
@@ -686,7 +688,9 @@ jobs:
     steps:
       - name: Install packages
         if: matrix.install
-        run: sudo apt install ${{matrix.install}}
+        run: |
+          sudo apt-get -o Acquire::Retries=$NET_RETRY_COUNT update
+          sudo apt-get -o Acquire::Retries=$NET_RETRY_COUNT -y ${{join(matrix.install, ' ')}}
 
       - name: Setup Boost
         run: |
@@ -760,8 +764,14 @@ jobs:
 
     steps:
       - name: Install packages
-        if: matrix.install
-        run: sudo apt install ${{matrix.install}}
+        run: |
+          if [ -f "/etc/debian_version" ]
+          then
+              sudo apt-get -o Acquire::Retries=$NET_RETRY_COUNT update
+              sudo apt-get -o Acquire::Retries=$NET_RETRY_COUNT install -y ninja-build ${{join(matrix.install, ' ')}}
+          else
+              brew install ninja ${{join(matrix.install, ' ')}}
+          fi
 
       - name: Setup Boost
         run: |
@@ -776,8 +786,7 @@ jobs:
           echo REF: $REF
           BOOST_BRANCH=develop && [ "$REF" = "master" ] && BOOST_BRANCH=master || true
           echo BOOST_BRANCH: $BOOST_BRANCH
-          # Building tests on Mac OS occasionally fail with a timeout if there are too many parallel build jobs
-          BUILD_JOBS=$((nproc || echo 1) 2> /dev/null)
+          BUILD_JOBS=$((nproc || sysctl -n hw.ncpu) 2> /dev/null)
           echo BUILD_JOBS: $BUILD_JOBS
           echo "BUILD_JOBS=$BUILD_JOBS" >> $GITHUB_ENV
           echo "CMAKE_BUILD_PARALLEL_LEVEL=$BUILD_JOBS" >> $GITHUB_ENV
@@ -807,7 +816,8 @@ jobs:
         run: |
           cd boost-root
           mkdir __build__ && cd __build__
-          cmake -DBOOST_INCLUDE_LIBRARIES=$LIBRARY -DBUILD_TESTING=ON ..
+          # Building tests on Mac OS using makefiles sometimes blocks until timeout, if multiple parallel build jobs are used. Use Ninja instead.
+          cmake -G Ninja -DBOOST_INCLUDE_LIBRARIES=$LIBRARY -DBUILD_TESTING=ON ..
 
       - name: Build tests
         run: |