Use mirrored buildx for PRs Only

[mikaelweave]

Description

The mirrored buildx is producing invalid multi-platform images. This isn't causing issues with PRs which build for a single platform. Changing logic to only use mirror for PRs.

Related issues

AB#135348

Testing

Running commands locally.

FHIR Team Checklist

• Update the title of the PR to be succinct and less than 65 characters
• Add a milestone to the PR for the sprint that it is merged (i.e. add S47)
• Tag the PR with the type of update: Bug, Build, Dependencies, Enhancement, New-Feature or Documentation
• Tag the PR with Open source, Azure API for FHIR (CosmosDB or common code) or Azure Healthcare APIs (SQL or common code) to specify where this change is intended to be released.
• Tag the PR with Schema Version backward compatible or Schema Version backward incompatible or Schema Version unchanged if this adds or updates Sql script which is/is not backward compatible with the code.
• CI is green before merge
• Review squash-merge requirements

Semver Change (docs)

Patch|Skip|Feature|Breaking (reason)

[mikaelweave]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[mikaelweave]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[mikaelweave]

/azp run

[azure-pipelines]

Azure Pipelines failed to run 1 pipeline(s).

[mikaelweave]

/azp run

[azure-pipelines]

Azure Pipelines failed to run 1 pipeline(s).