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 https://github.com/actions/checkout/pull/379
it was decided to keep the `for` loop in the interest of simplicity.

Reported by Ian Lynagh.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

src/git-auth-helper.ts

@@ -148,7 +148,7 @@ class GitAuthHelper {
         output.match(/(?<=(^|\n)file:)[^\t]+(?=\tremote\.origin\.url)/g) || []
       for (const configPath of configPaths) {
         core.debug(`Replacing token placeholder in '${configPath}'`)
-        this.replaceTokenPlaceholder(configPath)
+        await this.replaceTokenPlaceholder(configPath)
       }
 
       if (this.settings.sshKey) {