makebuildserver: avoid trying to continue cache downloads when checksum is okay
makebuildserver does a http head request for each and every file in cache and attempts verifying the checksum only after those requests finished.
Starting new HTTPS connection (1): dl.google.com:443
https://dl.google.com:443 "HEAD /android/repository/tools_r25.2.5-linux.zip HTTP/1.1" 200 0
...shasum verified for /home/uniq/.cache/fdroidserver/tools_r25.2.5-linux.zip
Starting new HTTPS connection (1): dl.google.com:443
https://dl.google.com:443 "HEAD /android/repository/android_m2repository_r47.zip HTTP/1.1" 200 0
...shasum verified for /home/uniq/.cache/fdroidserver/android_m2repository_r47.zip
Starting new HTTPS connection (1): dl.google.com:443
https://dl.google.com:443 "HEAD /android/repository/android-2.1_r03.zip HTTP/1.1" 200 0
...shasum verified for /home/uniq/.cache/fdroidserver/android-2.1_r03.zip
...
...
...
This can be improved by simply checking verifying early.