Check out SDL sources only once

This prevents weird issues where a commit can happen between when the
i386 binaries are built and when amd64 binaries are built, and also
speeds up the workflow a bit.

.github/workflows/build.yml

@@ -7,19 +7,22 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
+    - name: Check out SDL sources
+      run: |
+        hg clone https://hg.libsdl.org/SDL /tmp/SDL
     - name: Build i386 binaries
       run: |
         mkdir /tmp/i386
         cd linux-build
         docker build -t sdl2-i386 -f Dockerfile.i386 .
-        docker run -v `pwd`/compile:/entry -v /tmp/i386:/io sdl2-i386
+        docker run -v `pwd`/compile:/entry -v /tmp/i386:/io -v /tmp/SDL:/src sdl2-i386
         cp /tmp/i386/lib/libSDL2-2.0.so.0 ../native/linux-x86
     - name: Build amd64 binaries
       run: |
         mkdir /tmp/amd64
         cd linux-build
         docker build -t sdl2-amd64 -f Dockerfile.amd64 .
-        docker run -v `pwd`/compile:/entry -v /tmp/amd64:/io sdl2-amd64
+        docker run -v `pwd`/compile:/entry -v /tmp/amd64:/io -v /tmp/SDL:/src sdl2-amd64
         cp /tmp/amd64/lib/libSDL2-2.0.so.0 ../native/linux-x64
     - name: Create pull request
       uses: peter-evans/create-pull-request@v2

linux-build/compile

@@ -2,8 +2,8 @@
 
 set -euf -o pipefail
 
-hg clone https://hg.libsdl.org/SDL SDL
-cd SDL
+cd /src
+rm -rf build
 mkdir build
 cd build
 ../configure --prefix=/io