Respect sticky/setuid bits when unzipping docs remotely

The spring-integration/docs/ directory on static.springframework.org
has sticky group-write permissions.  unzip is now invoked with the '-K'
flag to ensure that newly created directories respect these permissions.

This is important to ensure that different (ssh) users can release the
project without running into permissions errors.  Without -K, the unzip
command will preserve whatever permissions were present on the files and
directories at the time of archiving, and this usually means that
group-write is off.

Of course, all this assumes that users doing releases are part of the same
group - 'springorg' in this case.

docs/build.gradle

@@ -225,7 +225,7 @@ uploadArchives {
                     classpath: configurations.scpAntTask.asPath)
 
             // copy the archive, unpack it, then delete it
-            def unpackCommand = "cd ${remoteDocsDir} && unzip -o ${archive.archiveName}"
+            def unpackCommand = "cd ${remoteDocsDir} && unzip -K -o ${archive.archiveName}"
             def deleteCommand = "rm ${remoteDocsDir}/${archive.archiveName}"
 
             println "sshexec ${unpackCommand}"