addPath always prepend to path

[jetersen]

Describe the enhancement
core.addPath
assumes you always wants to prepend, however in some cases, you want to append to path.

Appending is most likely the case for dotnet global tools.

Either add another method or have a boolean/option to choose whether you want to append/prepend

Additional information
actions/setup-dotnet#69 (comment)

[joshmgross]

Why do dotnet global tools need to append instead of prepend?

[jetersen]

there might be executable that could accidentally override the order of things.

[jimhester]

One example where appending is necessary is for the setup-r action. On macOS the action needs to install a gfortran .pkg, and this should really be appended to the PATH, because the gfortran pkg contains a gcov executable, but we want to continue to use the normal gcov executable shipped with llvm for most code.

As a workaround I put a call to core.addPath("/usr/bin") after the call adding the gfortran PATH, but that then breaks other actions (such as setup-python) which put tools before "/usr/bin".

Support for appending path in the actions toolkit would give a solution to this problem. we would just call core.addPath("path/to/gfortran", append = true)