prevent windows exec unintentional relative pathing

[secjosh]

Hi maintainers! I'm Josh and I work at Bison Trails. We're doing an internal hackathon this week.

Our team loves aws-vault, but it got flagged during a recent security review. Our team voted to use our hackathon time to patch vulnerabilities in our favorite OSS tools and specifically to try to get aws-vault approved for usage within our organization.

What's here:

• Prevent wrong executable for credential process and exec subcommand on windows by removing current working directory from the beginning of $PATH. see more here https://blog.golang.org/path-security

Impact:

These changes should improve aws-vault's security posture. We identified this vulnerability using Salus and the OSSF scorecard. The specific issues I am hoping to address here are:

• Preventing wrong executable from being run by replacing os/exec with golang.org/x/sys/execabs on all go files that are built for windows.

What's next:

• an additional PR for respecting scripts execute permissions on assumerolewithwebidentity

[mtibben]

Thanks @secjosh