Change SyncMap supported types check style

[loosebazooka]

Allow for future extension with more supported builders like
jib, which is expected to produce a syncMap

Just trying to get used to the skaffold code and go things with small PRs.

[codecov]

Codecov Report

Merging #3328 into master will not change coverage.
The diff coverage is 0%.

Impacted Files	Coverage Δ
pkg/skaffold/build/local/local.go	43.63% <0%> (ø)	⬆️

[balopat]

unrelated error:

--- FAIL: TestBuildKanikoInsecureRegistry (49.56s)
    build_test.go:374: skaffold build: exit status 1, Generating tags...
         - kaniko-insecure -> 34.70.129.71:5000/kaniko-insecure:v1.0.0-124-gceb1a88
        Checking cache...
         - kaniko-insecure: Not found. Building
        Creating kaniko secret [default/e2esecret]...
        Building [kaniko-insecure]...
        Storing build context at /tmp/context-06fa688fd5a564d4c05a6136236a8f8c.tar.gz
    panic.go:406: time="2019-12-02T23:58:37Z" level=fatal msg="build failed: build failed: building [kaniko-insecure]: kaniko build for [kaniko-insecure]: waiting for pod to complete: condition error: pod already in terminal phase: Failed"