SLSA BB.6: Isolated builds (Build L3)

The platform must run build steps in an environment free of unintended external influence, including between builds of the same tenant project; any outside influence must be requested by the build itself. It must guarantee that a build cannot reach platform secrets such as the provenance signing key; that builds running at the same time cannot influence each other (for example through shared memory); that a build cannot persist into or affect the environment of a later build, so each build gets a freshly provisioned ephemeral environment; that a build cannot plant false entries in a cache another build uses, so output is identical with or without the cache; and that the platform opens no service allowing remote influence unless every such interaction is recorded as an external parameter. The requirement covers only the platform's guarantees for a well-intended build: it does not stop a producer calling a remote execution service or self-hosted runner outside the trust boundary, and it is distinct from hermetic (no network) builds.