GitHub Actions: add Linux CI (in addition to existing BuildBot)

We already run CI since 2014, first via Travis CI, then when it became
paid we switched to self-hosted BuildBot in 2019. Later that year
GitHub Actions came also in existence, but we already switched over to
BuildBot by then so didn't use it.

We will still use BuildBot on self-hosted to test various Ubuntu and
Debian distro versions, FreeBSD and Windows. Also, in the BuildBot we
have our own pre-build environment where we run Services tests (with
both anope and atheme), we run TLS there (again on all those distros
with various OpenSSL versions). And we also test both clang and gcc.

So what is new? Well, now we will also run a "quick test" via GitHub
Actions, like most projects out there on GitHub. Not the services test,
not the TLS tests, but simply latest Ubuntu and then clang+gcc.
The main benefit of this is that it will also show up on Pull Requests
and makes it "public" as our BuildBot page is restricted.

.github/workflows/linux-ci.yml

@@ -0,0 +1,36 @@
+name: Linux CI
+
+on:
+  push:
+    branches: [ "unreal60_dev" ]
+  pull_request:
+    branches: [ "unreal60_dev" ]
+
+env:
+  NOSERVICES: 1
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        compiler: [ {c: gcc, cpp: g++}, {c: clang, cpp: clang++} ]
+
+    env:
+      CC: ${{ matrix.compiler.c }}
+      CXX: ${{ matrix.compiler.cpp }}
+
+    steps:
+    - name: installdependencies
+      run: |
+        sudo rm /var/lib/man-db/auto-update
+        sudo apt-get install build-essential pkg-config libssl-dev libpcre2-dev libargon2-dev libsodium-dev libc-ares-dev libcurl4-openssl-dev libjansson-dev
+    - name: installpythondependencies
+      run: |
+        python -m pip install --break-system-packages pyasyncore
+        python -m pip install --break-system-packages pyasynchat
+    - uses: actions/checkout@v4
+    - name: build
+      run: extras/build-tests/nix/build
+    - name: run-tests