Use USERPROFILE env variable on Windows

$HOME is not defined by default on Windows.
$USERPROFILE will return the desired value

fixes actions-rs/toolchain#185, actions-rs/toolchain#137

src/commands/rustup.ts

@@ -90,8 +90,8 @@ export class RustUp {
                 );
         }
 
-        // `$HOME` should always be declared, so it is more to get the linters happy
-        core.addPath(path.join(process.env.HOME!, '.cargo', 'bin')); // eslint-disable-line @typescript-eslint/no-non-null-assertion
+        // `$HOME` or `$USERPROFILE` should always be declared, so it is more to get the linters happy
+        core.addPath(path.join((process.env.HOME || process.env.USERPROFILE)!, '.cargo', 'bin')); // eslint-disable-line @typescript-eslint/no-non-null-assertion
 
         // Assuming it is in the $PATH already
         return new RustUp('rustup');