go.mod: bump github.com/bradfitz/go-tool-cache

* github.com/bradfitz/go-tool-cache: v0.0.0-20260909201542-a1c7321be47b to v0.0.0-20260919185303-c660171c910c

Triggered by @bradfitz via the bumpdep workflow.

Updates tailscale/corp#47471

Signed-off-by: Dep Updater <noreply+dep-updater@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f