fix custom script handling for node/python/ruby/dotnet entry points

[scott-es]

Expected behaviour

when using .snyk.d/prep.sh for node/python/ruby/dotnet, run_snyk is executed so that any custom commands are done before the rest of the script is executed.

Actual behaviour

when using .snyk.d/prep.sh for node, run_snyk is not executed. In the the language entrypoints, the custom script will run but run_snyk is also still called.

[scott-es]

target dates to resolve this:
node 6/6/22 #57
python 6/9/22
ruby 6/13/22
dotnet 6/20/22