Add --enable-debug flag to musl CI build script

Add --enable-debug flag to musl CI build script

Building for x86_64-unknown-linux-musl currently results in an executable lacking debug information for musl libc itself. If you request a backtrace in GDB while control flow is within musl – including sycalls made by musl – the result looks like:

#0  0x0000000000434b46 in __cp_end ()
#1  0x0000000000432dbd in __syscall_cp_c ()
#2  0x0000000000000000 in ?? ()

i.e. not very helpful. Adding --enable-debug resolves this, and --enable-optimize re-enables optimisations which default to off given the previous flag.