Document new env var

Author: oli-obk

src/tools/miri/README.md

@@ -478,6 +478,8 @@ Moreover, Miri recognizes some environment variables:
 * `MIRI_TEST_TARGET` (recognized by the test suite and the `./miri` script) indicates which target
   architecture to test against.  `miri` and `cargo miri` accept the `--target` flag for the same
   purpose.
+* `MIRI_TEST_THREADS` (recognized by the test suite): set the number of threads to use for running tests.
+  By default the number of cores is used.
 * `MIRI_NO_STD` (recognized by `cargo miri` and the test suite) makes sure that the target's
   sysroot is built without libstd. This allows testing and running no_std programs.
 * `RUSTC_BLESS` (recognized by the test suite and `cargo-miri-test/run-test.py`): overwrite all