build: Correct the CI action

* Move build folder to env.
* Reduce verbosity.
* Do CI only on pushes to master to reduce the load on environment when
I do pull requests from the branches in this repo (not from forks).
* Utilize both CPU cores for faster build.
* Shorten name of the CI action.

Signed-off-by: Alexander Kurbatov <Alexander.Kurbatov@acronis.com>

Author: Alexander Kurbatov

.github/workflows/crossplatform-ci.yml renamed to .github/workflows/ci.yml

@@ -1,7 +1,9 @@
-name: Crossplatform CI
+name: CI
 
 on:
   push:
+    branches:
+      - master
     paths:
       - 'src/**'
       - 'contrib/**'
@@ -14,6 +16,7 @@ on:
 
 env:
   BUILD_TYPE: Release
+  BUILD_DIR: ${{github.workspace}}/build
 
 jobs:
   build:
@@ -28,9 +31,7 @@ jobs:
         submodules: recursive
 
     - name: Configure
-      run: cmake -B ${{github.workspace}}/build -DCMAKE_BUILD_TYPE=${{env.BUILD_TYPE}}
+      run: cmake -B ${{env.BUILD_DIR}} -DCMAKE_BUILD_TYPE=${{env.BUILD_TYPE}}
 
     - name: Build
-      run: cmake --build ${{github.workspace}}/build --config ${{env.BUILD_TYPE}}
-      env:
-        VERBOSE: 1
+      run: cmake --build ${{env.BUILD_DIR}} --config ${{env.BUILD_TYPE}} --parallel 2