[mtu-pr79-#80221576]: Work around backwards-incompatible fly changes

 - Must now use -p to ensure tests run as root
 - Path element in build.yml doesn't work, so copy to $GOPATH manually

[#80118996]
[mtu-pr79-#80221576]: Update DEVELOPING.md

 - take out extraenous colon
 - no need to build garden to test it

[#80118996]
[mtu-pr79-#80221576]: Fix path error in .gitignore (after moving stuff to old/...).
Cherry-pick branch 'your-fly-is-undone' from Julz and Glyn

* Update DEVELOPING.md
* Work around backwards-incompatible fly changes

also

* Fix path error in .gitignore (after moving stuff to old/...)
* add scripts/garden-fly to capture the necessary fly parameter(s)
* updated README.md to describe testing under fly