Add main (1.19) and main FIPS tags

[dagood]

Add microsoft/go main builds to the microsoft/go-images microsoft/nightly branch. Add FIPS tags that build on top of them and set GOEXPERIMENT=opensslcrypto by default.

• For proper manifest.json generation, depends on: Support fips-linux variant and main for microsoft/go-images go-infra#45
• Part of Integrate FIPS changes into microsoft/main go#501

The plan is to only build main tags in this repo's microsoft/nightly branch, not the MAR/MCR-targeting microsoft/main branch. Builds that come out of microsoft/go's microsoft/main branch never belong on MAR, only in our public ACR for evaluation purposes.

Once 1.19 is released and there's a release branch, the FIPS tags will be included in MAR builds.

Having go-images produce nightly builds based on microsoft/main will help us make sure our microsoft/main changes are ready for the coming 1.19 release. In the future it will help with 1.20, etc.

Review tips:

• The Dockerfile diffs are not loaded by default because they're generated--I suggest taking a peek at one from each variant type to see the templates in action.
• The manifest.json is generated from versions.json. Support fips-linux variant and main for microsoft/go-images go-infra#45 has more context on the details in this file.

Example internal build: https://dev.azure.com/dnceng/internal/_build/results?buildId=1806155&view=results

[qmuntal]

LGTM!