CI: Add a GitHub Action to build on Ubuntu 20.04 LTS

Also clean up MSYS2 action.

.github/workflows/build_linux.yml

@@ -0,0 +1,44 @@
+name: build-linux
+
+on:
+  push:
+  pull_request:
+
+env:
+  BUILD_TYPE: Release
+
+jobs:
+  build:
+    runs-on: ubuntu-20.04
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+      with:
+        fetch-depth: 0
+
+    - name: Setup
+      shell: bash
+      run: |
+        sudo apt-get update
+        sudo apt-get install -y build-essential    \
+                           gcc-10                  \
+                           g++-10                  \
+                           pkg-config              \
+                           cmake                   \
+                           make                    \
+                           libglfw3-dev            \
+                           libglm-dev              \
+                           libmagic-dev            \
+                           libssl-dev              \
+                           libcapstone-dev         \
+                           llvm-dev                \
+                           nlohmann-json3-dev      \
+                           python3-dev             \
+
+    - name: Build
+      shell: bash
+      run: |
+        mkdir build
+        cd build
+        CC=gcc-10 CXX=g++-10 cmake ..
+        make -j 4