Use $0 instead of $_ for sysroot inference

[danielocfb]

The reliance on $_ seems wrong, as it leads to wrong sysroot inference when one of the wrapper scripts is invoked in the context of another program. E.g.,

$ timeout 10s bpftrace -e 'uprobe:/system/lib64/libc.so:malloc { @ = hist(arg0); }'
/vendor/bin/bpftrace[5]: /system/bin/run.sh: inaccessible or not found

As can be seen, the sysroot is inferred to be /system/ in this case (the directory in which timeout resides), which is not correct (everything bpf is located below /vendor).

With this change we use $0 instead of $_ to fix this problem.

[michalgr]

Thanks !

[danielocfb]

Thanks for merging so quickly.