This commit introduces new CI scripts and environment files for running Bazel CPU presubmits.
* Adds a ci directory at the root of the repository to store these files.
* Environment files are located in ci/envs and define new JAXCI_ environment variables to control CI build behavior.
* The build script sources these environment files and set up the build environment before running the build commands.
PiperOrigin-RevId: 695957540
