Reliance upon HOME environment variable

[mparry]

There are a couple of places (src/exec.ts and src/resources.ts) that rely upon the existence of a HOME environment variable. On Windows this will not exist by default. I think you should probably also be trying USERPROFILE. For example, I got it working by editing those files as follows (respectively):

function helmHome() {
    return process.env["HELM_HOME"]
         || filepath.join(process.env["HOME"] || process.env["USERPROFILE"], '.helm');
}

let home = shell.env["HOME"] || shell.env["USERPROFILE"];

I've now added a PR (see below) that makes the same change.