Add option to specify julia path

[NilsWildt]

Hi, It seems that on my Windows system, I'm having a hard time to specify the PATH -- such that it would find the correct julia executable (in _get_script). Would it be hard to add a kwarg option with a julia executable path?

[MilesCranmer]

Good point!

Even better would be if we could change this line:

AirspeedVelocity.jl/src/Utils.jl

Line 219 in 9c9cce7

run(`julia --project="$tmp_env" --startup-file=no $exeflags "$runner_filename"`)

to use the code used in Pkg.test() which is like:

https://github.com/JuliaLang/julia/blob/147bdf428cd14c979202678127d1618e425912d6/base/util.jl#L677-L680

run(`$(julia_cmd()) $(joinpath(Sys.BINDIR, Base.DATAROOTDIR, "julia", "test", "runtests.jl")) $tests`, ENV2))

i.e., it uses the julia_cmd() to get the right binary to call.

I'm also wondering if we should just use a Distributed.process instead. But it's interesting that Pkg.test() itself uses a regular run().

x-ref related #21