(instead of once per getenv call)

Also, convert all shell scripts to POSIX-compliant sh scripts and apply
most shellcheck suggestions.

The previous build & test setup was invalid: So far we have built
_different_ libdash0envhook.so binaries for the LD_PRELOAD hook per
libc flavor. With this, the binary works in the target system.

But actually this approach is not not possible in the real world. We
can set one binary as LD_PRELOAD when instrumenting a container and we
do not know which libc flavor it is ahead of time. Thus we ultimately
need to build a libdash0envhook.so binary that works independently of
which libc flavor the target system uses.

This commit only fixes the test setup. The binary does not yet support
both libc flavors. This will be handled in a follow up commit.

[skip ci]

[skip ci]