Add notify action with nixpkgs update

[kachick]

Resolves GH-846

Preparation for GH-902

[kachick]

1. Clone with depth 1
2. shallow since target commit. Do not depend on fixed datetime

[kachick]

GraphQL: Resource not accessible by integration (addComment)

cli/cli#8374

Didn't work in PR by default?

[kachick]

Raising same error as #903 (comment) after merge 🤔

[kachick]

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[kachick]

Followed by these commits 85ee29b...c2cddc8