Don't use absolute path for signtool in sign_binaries.py

[RyanJarv]

The env var checked before doesn't seem to get passed to this script and the fallback signtool path doesn't exist in a standard visual studios setup.

There's likely a better way to do this but this with setting the path before hand is the simplest fix for now. During the build it will look something like this:

$env:PATH = "C:\Program Files (x86)\Windows Kits\10\bin\10.0.15063.0\x64\;$env:PATH"
yarn run create_dist Release --debug_build=false --official_build=false --channel=beta

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.

Test Plan:

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions

[bridiver]

actually shouldn't this be 'signtool {}'.format(signtool_args)? I guess both ways work?

[RyanJarv]

yeah that's not intentional, will fix this.

[RyanJarv]

@bridiver mind taking a look at this again?

[RyanJarv]

This allows us to work around the this issue brave/brave-browser#323.