ci: refactor ci so common args are picked from one location

Mostly. It is annoying if we want to add some flag/option to all the cis
and then have to do it manually. Things easily get missed that way.
Skipped powershell because of course I'm not going to deal with that.

ci/build-linux-old.sh

@@ -1,6 +1,8 @@
 #!/bin/sh
 set -e
 
+. ./ci/build-common.sh
+
 # clone exactly the oldest libplacebo we want to support
 rm -rf subprojects
 mkdir -p subprojects
@@ -8,12 +10,8 @@ git clone https://code.videolan.org/videolan/libplacebo.git \
     --recurse-submodules --shallow-submodules \
     --depth=1 --branch v6.338 subprojects/libplacebo \
 
-meson setup build \
-    --werror      \
-    -Dlibplacebo:vulkan=disabled \
-    -Dlibmpv=true \
-    -Dlua=enabled \
-    -Dtests=true
-
+meson setup build $common_args \
+ -Dlibplacebo:vulkan=disabled \
+ -Dlua=enabled
 meson compile -C build
 ./build/mpv -v --no-config