Update sigstore/cosign-installer action to v2.8.1

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
sigstore/cosign-installer	action	minor	v2.6.0 -> v2.8.1

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Release Notes

sigstore/cosign-installer

### [`v2.8.1`](https://togithub.com/sigstore/cosign-installer/releases/tag/v2.8.1) [Compare Source](https://togithub.com/sigstore/cosign-installer/compare/v2.8.0...v2.8.1) #### What's Changed - bump cosign install to use release v1.13.1 by [@​cpanato](https://togithub.com/cpanato) in [https://github.com/sigstore/cosign-installer/pull/98](https://togithub.com/sigstore/cosign-installer/pull/98) **Full Changelog**: https://github.com/sigstore/cosign-installer/compare/v2...v2.8.1 ### [`v2.8.0`](https://togithub.com/sigstore/cosign-installer/releases/tag/v2.8.0) [Compare Source](https://togithub.com/sigstore/cosign-installer/compare/v2.7.0...v2.8.0) #### What's Changed - bump cosign to v1.13.0 by [@​cpanato](https://togithub.com/cpanato) in [https://github.com/sigstore/cosign-installer/pull/95](https://togithub.com/sigstore/cosign-installer/pull/95) **Full Changelog**: https://github.com/sigstore/cosign-installer/compare/v2.7.0...v2.8.0 ### [`v2.7.0`](https://togithub.com/sigstore/cosign-installer/releases/tag/v2.7.0) [Compare Source](https://togithub.com/sigstore/cosign-installer/compare/v2.6.0...v2.7.0) #### What's Changed - bump cosign to v1.12.1 by [@​cpanato](https://togithub.com/cpanato) in [https://github.com/sigstore/cosign-installer/pull/94](https://togithub.com/sigstore/cosign-installer/pull/94) **Full Changelog**: https://github.com/sigstore/cosign-installer/compare/v2...v2.7.0

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