Set build hash to Unknown when unavailable

This commit changes the build hash to be the string "Unknown" when for
some reason this build hash is not available. This aligns the value with
the value we use when the hash is not available from the jar
manifest. This situation can occur when running tests from a worktree
which is not currently handled correctly by JGit, the upstream
dependency that is used to acquire the hash. This causes problems when
running tests locally because the warning header pattern expects a hash
or the string "Unknown". While the warning header pattern be changed to
allow "N/A" as well, it seemed more sensible to align the value here
with the value when the hash is not available from the jar manifest.

Relates #23421

buildSrc/src/main/groovy/org/elasticsearch/gradle/BuildPlugin.groovy

@@ -464,7 +464,7 @@ class BuildPlugin implements Plugin<Project> {
                         'Build-Java-Version': project.javaVersion)
                 if (jarTask.manifest.attributes.containsKey('Change') == false) {
                     logger.warn('Building without git revision id.')
-                    jarTask.manifest.attributes('Change': 'N/A')
+                    jarTask.manifest.attributes('Change': 'Unknown')
                 }
             }
         }