Shim: Use privately installed build tools

We currently install several external tools as part of the k8shim build:

• shellcheck (installs to ./shellcheck)
• golangci-lint (clobbers system version)
• kubectl (clobbers /usr/local/bin/kubectl via sudo)
• kind (clobbers $GOPATH/bin/kind)
• helm (clobbers system version)
• spark (installs to ./_spark)

We should refactor the build system to install all of these tools under ./tools (which will be .gitignored) and use these versions exclusively. This makes the used versions consistent and avoids clobbering other installed versions.