Perform partial build if BUILD_REFNAME is HEAD

[sjohnr]

Expected behavior

When the BUILD_REFNAME is HEAD (i.e. a local docs build), a partial build should be performed, since building the docs locally does not impact the remote site and users may have no control over the remote site.

Current behavior

When performing a local docs build, we receive the following warning:

> Task :spring-security-docs:antora
partial-build-extension: version 6.4.0 not previously built; reverting to full build 

because at the time, the site manifest was missing a 6.4.0 version. This meant we could not build the docs locally until after the docs-build branch performed a full build of the remote site.

Related gh-24