Fix multirun execution of scripts

[saksmt]

Fixes this:

multirun(
    name = "a",
    commands = [],
    parallel = False
)

multirun(
    name = "b",
    commands = ["//:a"],
    parallel = False
)

# results in: failed to start process for "a.bash": exec: "a.bash": executable file not found in $PATH

[saksmt]

So there is no CI...
Can anyone check if this actually works with existing tests? (have a problem running go under nixos)

Also need some guidance to how tests work and how to add this test case

[ash2k]

Thanks for the contribution! The builds are there but cannot report back. I've created #84 to look into it properly. Builds for your branch are here https://buildkite.com/bazel/atlassian-bazel-tools/builds?branch=saksmt%3Abugfix%2Fmultirun-bash-execution

Please see how tests are written in multirun/test directory. The idea is to run something simple that produces some output and compare it with expected output.

[ash2k]

I think a better approach would be to resolve relative paths in the instructions file taking the directory of that file as the base.

[ash2k]

I've tried this but it's not that straightforward because the path of the instruction file is relative to the exec root (or whatever) and it's just not worth the effort to do this.

[ash2k]

I'm closing this in favor of #86. Thank you for reporting the issue!