Run bazel in a distinct output base

[cameron-martin]

This prevents concurrent builds from blocking the language server.

See:

• https://github.com/bazelbuild/vscode-bazel/blob/cfc931cc88681774e832669c684bda2b65e9b90f/src/bazel/bazel_query.ts#L124-L132
• https://bazel.build/run/scripts#output-base-option