GitHub CI [Linux ARM]: use code checked out by GitHub

Use code checked out by GitHub Action - previously there was a clone
but we want rather the actual code, which doesn't need to be in master
branch of the main repository.

This also requires a small refactor (chroot in home rather than in
$GITHUB_WORKSPACE)

.github/scripts/Linux/arm/build.sh

@@ -7,21 +7,11 @@ export LD_LIBRARY_PATH=$EXTRA_LIB_PATH${LD_LIBRARY_PATH:+":$LD_LIBRARY_PATH"}
 export PATH=/usr/local/bin:$PATH
 export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig:${PKG_CONFIG_PATH:+":$PKG_CONFIG_PATH"}
 
-PWD=`pwd`
 APPDIR=UltraGrid.AppDir
 ARCH=`dpkg --print-architecture`
 APPNAME=UltraGrid-latest-${ARCH}.AppImage
 DIR=UltraGrid-AppImage
 
-cleanup() {
-        rm -rf $PWD/UltraGrid
-}
-trap cleanup EXIT
-
-git clone --depth 1 https://github.com/CESNET/UltraGrid.git
-
-cd UltraGrid
-
 ./autogen.sh --enable-plugins
 
 mkdir $APPDIR