Bump anyhow from 1.0.39 to 1.0.40 in /cmd/pinniped-proxy (#2614)

Bumps [anyhow](https://github.com/dtolnay/anyhow) from 1.0.39 to 1.0.40.
- [Release notes](https://github.com/dtolnay/anyhow/releases)
- [Commits](dtolnay/anyhow@1.0.39...1.0.40)

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