Use Go style pseudo-versions when there is no semver supplied

[negz]

Description of your changes

The CI workflow uses a semver (e.g. v1.0.0) when you run it using the workflow_dispatch trigger and supply one explicitly. This is how you 'release' a function.

Today when the workflow runs for a PR or regular main branch build we default to v0.0.0-<git-short-sha> (e.g. v0.0.0-9be3d00). This is a very simple way to tag development builds with no user input required. It's handy that the resulting package can be correlated to the git commit that produced it using the git SHA.

A major flaw of this implementation is that newer SHAs do not sort above older SHAs when treated as semantic versions.

This commit switches us to a simple approximation of the Go pseudoversions you would see in a go.mod file, as described by https://go.dev/ref/mod#pseudo-versions. These versions include the git commit time before the SHA, so newer commits will be considered newer versions when processing the semver. We also switch to using the first 12 characters of the SHA. This reduces the risk of collision, and makes us match exactly Go's pseudoversion implementation.

Development packages will now be pushed with a tag like v0.0.0-20231101115142-1091066df799.

I have:

• Read and followed Crossplane's contribution process.
• Added or updated unit tests for my change.

[stevendborrelli]

LGTM

[negz]

Confirmed the semantic version library we use will sort these correctly: https://go.dev/play/p/awYl-N0CzD4

[negz]

Also tested by updating function-auto-ready in crossplane-contrib/function-auto-ready#11. It works as expected:

2023-11-01T23:02:21Z	DEBUG	Wrote OCI index	{"ref": "xpkg.upbound.io/crossplane-contrib/function-auto-ready:v0.0.0-20231101225931-60aea09d94c8", "manifests": 2}