API builds cancel each other

[kaspar030]

e.g.,

• user A starts an API build A on branch master with extra env "FOO=bar"
• build starts
• user B starts an API build B on branch master with extra env "FOO=foobar"
• build B cancels build A

[kaspar030]

maybe there needs to be some "key", or make this user namespaced (builds only cancel old builds if they share the same key, or were started by the same user, or both).