Fix stray \ before + error caused by grep in load_eessi_extend_module.sh

[bedroge]

While inspecting the log of a build done in #27, I found this warning:

grep: warning: stray \ before +

It's caused by a grep in load_eessi_extend_module.sh, and can be reproduced with the grep from our compat layer:

$ /cvmfs/software.eessi.io/versions/2023.06/compat/linux/x86_64/startprefix 
Entering Gentoo Prefix /cvmfs/software.eessi.io/versions/2023.06/compat/linux/x86_64

$ echo "+++ b/EESSI-extend" | grep "^\+\+\+ b/"
grep: warning: stray \ before +
+++ b/EESSI-extend

But it shouldn't be necessary to escape the +:

$ echo "+++ b/EESSI-extend" | grep "^+++ b/"
+++ b/EESSI-extend

[ocaisa]

LGTM