Update bin/jupyter-kernel-gap

[fingolfin]

To find GAP, we now proceed as follows: First, if $JUPYTER_GAP_EXECUTABLE is set, we use that. If not, then we check if $GAP is set and use that. If not, we try to use gap in the current path.

Also avoid the use of which, which is non-standard. Instead, just hard code the name gap, and don't bother performing any further checks (we don't validate the value of $JUPYTER_GAP_EXECUTABLE or $GAP either)

Finally, use exec to start GAP, replacing the shell, instead of creating a fresh process.

The script now passes the tests on https://www.shellcheck.net

[pedritomelenas]

Perfect! Thanks!

[fingolfin]

Tests fail, I'll look into it.

[codecov[bot]]

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