Force specific hash when building a single docs branch

We tried this in #44204 but it doesn't seem to work.

The docs build was triggered for main and tried to reset to 57b44b05949404015d5935899cfd827665fdb093. Antora failed due to these warnings:

[09:05:32.609] WARN (asciidoctor): configuration property not found: server.tomcat.servlet.use-relative-redirects
    file: documentation/spring-boot-docs/src/docs/antora/modules/how-to/pages/webserver.adoc:565
    source: /home/runner/work/spring-boot/spring-boot/.git (branch: main | start path: documentation/spring-boot-docs/src/docs/antora)
[09:05:32.610] WARN (asciidoctor): configuration property not found: server.tomcat.servlet.redirect-context-root
    file: documentation/spring-boot-docs/src/docs/antora/modules/how-to/pages/webserver.adoc:565
    source: /home/runner/work/spring-boot/spring-boot/.git (branch: main | start path: documentation/spring-boot-docs/src/docs/antora)

The references to server.tomcat.servlet.use-relative-redirects and server.tomcat.servlet.redirect-context-root were not added until f50c8311b90b57e350e64f233c6a2daa72544a7c so it would appear that reseting to 57b44b05949404015d5935899cfd827665fdb093 did not work.

Start by reviewing issue #44204 and the two linked GitHub Actions runs to understand how the single-branch docs build selects its revision. Inspect the docs build configuration and the checkout/reset step, then verify that building at 57b44b05949404015d5935899cfd827665fdb093 does not read the later references from webserver.adoc:565.