debian: Add salsa-ci

This disable blhc since those currently fail with

$ blhc --debian --line-numbers --color ${SALSA_CI_BLHC_ARGS} ${WORKING_DIR}/*.build || [ $? -eq 1 ] 1272:CPPFLAGS missing (-D_FORTIFY_SOURCE=2): gcc -o checkkeys ../../test/checkkeys.c -g -O2 -ffile-prefix-map=/builds/agx/libsdl2/debian/output/source_dir=. -fstack-protector-strong -Wformat -Werror=format-security -I/builds/agx/libsdl2/debian/output/source_dir/include -DHAVE_OPENGLES2 -DHAVE_OPENGL -g -lSDL2_test -L/builds/agx/libsdl2/debian/output/source_dir/build/.libs -lSDL2 1272:LDFLAGS missing (-Wl,-z,relro): gcc -o checkkeys ../../test/checkkeys.c -g -O2 -ffile-prefix-map=/builds/agx/libsdl2/debian/output/source_dir=. -fstack-protector-strong -Wformat -Werror=format-security -I/builds/agx/libsdl2/debian/output/source_dir/include -DHAVE_OPENGLES2 -DHAVE_OPENGL -g -lSDL2_test -L/builds/agx/libsdl2/debian/output/source_dir/build/.libs -lSDL2 ...

Local testing is great but i always feel a bit better when salsa is o.k. with MRs too.