Cache option for Windows

[Fydon]

Description

Using git config --global gitsign.connectorID https://github.com/login/oauth with gitsign and then perform multiple commits can result in reaching the secondary rate limit on calling the GitHub repository. Current cache option doesn't work on Windows as mention in #194.

Version

gitsign version v0.10.1
parsed config:
{
  "Fulcio": "https://fulcio.sigstore.dev",
  "FulcioRoot": "",
  "Rekor": "https://rekor.sigstore.dev",
  "RekorMode": "online",
  "ClientID": "sigstore",
  "RedirectURL": "",
  "Issuer": "https://oauth2.sigstore.dev/auth",
  "ConnectorID": "https://github.com/login/oauth",
  "TokenProvider": "",
  "TimestampURL": "",
  "TimestampCert": "",
  "LogPath": "",
  "CommitterName": "Foo",
  "CommitterEmail": "Bar",
  "MatchCommitter": false,
  "Autoclose": true,
  "AutocloseTimeout": 6
}

[wlynch]

I'm open to this, but I do not have any sort of Windows socket experience (or much experience building Windows apps in general). 😓

The first step would be to hide unix-specific syscalls behind a build flag:

$ GOOS=windows go build ./cmd/gitsign-credential-cache
# github.com/sigstore/gitsign/cmd/gitsign-credential-cache
./main.go:43:10: undefined: syscall.Umask