CI ARM: Buster armhf compatibility

Allow 32-bit builds on Buster. This has some advantages
regarding supported HW accelerations in UltraGrid, eg. h264_omx is not
supported in higher versions of the system.

It is not enabled by default but can be enabled easily by switching the
value in YAML file.

refers to GH-344

.github/scripts/Linux/arm/bootstrap.sh

@@ -1,6 +1,7 @@
 #!/bin/sh -exu
 
-readonly debver=bullseye
+debver=$(. /etc/os-release; echo "$VERSION_CODENAME")
+readonly debver
 
 apt -y install curl gnupg
 echo -k > ~/.curlrc
@@ -20,7 +21,14 @@ http://archive.raspberrypi.org/debian $debver main
 EOF
 apt -y update
 
-apt -y install autoconf automake build-essential cmake git pkg-config libtool sudo
+if [ "$debver" = buster ]; then
+        # 3.16 in the added repository is broken with chrooted qemu-user-static
+        apt -y install cmake=3.13.4-1 cmake-data=3.13.4-1
+else
+        apt -y install cmake
+fi
+
+apt -y install autoconf automake build-essential git pkg-config libtool sudo
 apt -y install libcurl4-openssl-dev libsoxr-dev libspeexdsp-dev libssl-dev
 apt -y install libasound2-dev portaudio19-dev libjack-dev
 apt -y install libglew-dev libglfw3-dev libglm-dev