Allow overriding Buf version in PATH

[emcfarlane]

This avoids requiring the version of Buf found in the PATH to match the resolved. Fallback to installing a new explicit version in the toolcache.

Fixes #71

[nicksnyder]

Does this mean a second install overwrites the first install? Is this going to solve the problem for the user or do we need to be stamping the version number in the binary name so we can have multiple binaries installed at once?

[emcfarlane]

@nicksnyder adding more binaries to the tool cache is stored by version/arch (see the cachePath). So installing multiple will work as intended. The current environment PATH will always be updated by prepending the latest installed. So buf will resolve to the first version found in the PATH. Node installs in this way and adds the binary to the path (see the function setupNodeJs in the node action).

For the users issue, they will install multiple versions of buf extending the PATH environment for the current run. On a new instantiation of a worker, the PATH will not include the toolcache location of the multiple buf versions. The action, buf-action, will then resolve the version found in the toolcache and add that to the PATH. This cache is the speed benefit the user was requesting support for.

[nicksnyder]

Cool, thanks for clarifying