* auth-helper: properly await replacement of the token value in the config

After writing the `.extraheader` config, we manually replace the token
with the actual value. This is done in an `async` function, but we were
not `await`ing the result.

In our tests, this commit fixes a flakiness we observed where
`remote.origin.url` sometimes (very rarely, actually) is not set for
submodules. Our interpretation is that the configs are in the process of
being rewritten with the correct token value _while_ another `git
config` that wants to set the `insteadOf` value is reading the config,
which is currently empty.

A more idiomatic way to fix this in Typescript would use
`Promise.all()`, like this:

      await Promise.all(
        configPaths.map(async configPath => {
          core.debug(`Replacing token placeholder in '${configPath}'`)
          await this.replaceTokenPlaceholder(configPath)
        })
      )

However, during review of #379
it was decided to keep the `for` loop in the interest of simplicity.

Reported by Ian Lynagh.

Signed-off-by: Johannes Schindelin <[email protected]>

* downloadRepository(): await the result of recursive deletions

Signed-off-by: Johannes Schindelin <[email protected]>

* Ask ESLint to report floating Promises

This rule is quite helpful in avoiding hard-to-debug missing `await`s.

Note: there are two locations in `src/main.ts` that trigger warnings:
the `run()` and the `cleanup()` function are called without `await` and
without any `.catch()` clause.

In the initial version of #379,
this was addressed by adding `.catch()` clauses. However, it was
determined that this is boilerplate code that will need to be fixed in a
broader way.

Signed-off-by: Johannes Schindelin <[email protected]>

* Rebuild

This trick was brought to you by `npm ci && npm run build`. Needed to
get the PR build to pass.

Signed-off-by: Johannes Schindelin <[email protected]>