feat: Update upgrade-matrix.yaml in post-upgrade based on env variable

[lukeogg]

What problem does this PR solve?:

Updates upgrade-matrix.yaml in post-release with the value from an environment variable. The variable is set in gh-dkp

https://github.com/mesosphere/gh-dkp/pull/492 needs to be merged first.

Which issue(s) does this PR fix?:

https://jira.nutanix.com/browse/NCN-100509

Special notes for your reviewer:

Test manually with:

export NKP_RELEASE_AUTOMATION_UPGRADE_MATRIX="upgrades:"
go run . post-release --version v2.21.31 --repo /Users/luke.ogg/Projects/kommander-applications

Does this PR introduce a user-facing change?:

Checklist

• If the PR adds a version bump, ensure there is no breaking change in Licensing model (or NA).
• If a chart is changed or app configuration is significantly changed, the chart version is correctly incremented (so that apps are not automatically upgraded from a previous version of DKP).

[coveralls]

Pull Request Test Coverage Report for Build 10950762331

Details

• 13 of 25 (52.0%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.04%) to 51.557%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
hack/release/pkg/upgradematrix/upgradematrix.go	13	18	72.22%
hack/release/cmd/postrelease/postrelease.go	0	7	0.0%

Files with Coverage Reduction	New Missed Lines	%
hack/release/cmd/postrelease/postrelease.go	1	0.0%

Totals
Change from base Build 10947650573:	0.04%
Covered Lines:	149
Relevant Lines:	289

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💛 - Coveralls