.github/workflows: start ninja with the "-C" option

Let's make this a single command, removing the overhead for two "cd"
commands (the latter of which had no effect anyway because it was the
last command in the shell process).

.github/workflows/build_linux_x86.yml

@@ -93,7 +93,7 @@ jobs:
               -DCMAKE_C_COMPILER_LAUNCHER=ccache -DCMAKE_CXX_COMPILER_LAUNCHER=ccache \
               -S ./ -B ./build
     - name: Ninja
-      run: cd build && ninja && cd ..
+      run: ninja -C build
     - name: Create package
       run:  |
             mkdir accounts logs save scripts

.github/workflows/build_linux_x86_64.yml

@@ -51,7 +51,7 @@ jobs:
               -DCMAKE_C_COMPILER_LAUNCHER=ccache -DCMAKE_CXX_COMPILER_LAUNCHER=ccache \
               -S ./ -B ./build
     - name: Ninja
-      run: cd build && ninja && cd ..
+      run: ninja -C build
     - name: Create package
       run:  |
             mkdir accounts logs save scripts

.github/workflows/build_osx_arm.yml

@@ -45,7 +45,7 @@ jobs:
             mkdir -p build
             cmake -G "Ninja" -DCMAKE_BUILD_TYPE="Nightly" -DCMAKE_TOOLCHAIN_FILE="$CMAKE_TCH" -S . -B ./build
     - name: Ninja
-      run: cd build && ninja && cd ..
+      run: ninja -C build
     - name: Create package
       run:  |
             pwd

.github/workflows/build_osx_x86_64.yml

@@ -43,7 +43,7 @@ jobs:
             mkdir -p build
             cmake -G "Ninja" -DCMAKE_BUILD_TYPE="Nightly" -DCMAKE_TOOLCHAIN_FILE="$CMAKE_TCH" -S . -B ./build
     - name: Ninja
-      run: cd build && ninja && cd ..
+      run: ninja -C build
     - name: Create package
       run:  |
             pwd