Unable to install rustfmt for a custom toolchain

Problem
I'm building a custom rust toolchain to support bitcode under iOS using the scripts from: https://github.com/cowpaths/rust-bitcode/tree/fs-nightly-2019-09-05-llvm8 (which is a slightly modified fork of https://github.com/getditto/rust-bitcode )

The toolchain was installed using the command:

rustup toolchain link fs-2019-09-05-llvm8 ~/.fs-rust/toolchain-fs-2019-09-05-llvm8
rustup default fs-2019-09-05-llvm8

and everything seems to be fine. I can compile for iOS and for macosx using cargo build and cargo run. Now I'd like to install rustfmt. When I try:

rustup component add rustfmt

I get the following output:

error: toolchain 'fs-2019-09-05-llvm8' does not support components
error: caused by: invalid toolchain name: 'fs-2019-09-05-llvm8'

So my question is, what do I need to do to install rustfmt (and the next thing woul;d be clippy).

Steps
See above

Notes

Output of rustup --version:
rustup 1.20.2+125 (5f6317e9f 2019-12-03)

Output of rustup show:

Default host: x86_64-apple-darwin
rustup home:  /Users/davehylands/.rustup

installed toolchains
--------------------

stable-x86_64-apple-darwin
nightly-2019-09-05-x86_64-apple-darwin
nightly-2019-09-13-x86_64-apple-darwin
nightly-x86_64-apple-darwin
fs-2019-09-05-llvm8 (default)
ios-arm64
1.37.0-x86_64-apple-darwin
1.38.0-x86_64-apple-darwin

active toolchain
----------------

fs-2019-09-05-llvm8 (default)
rustc 1.39.0-dev

Reproduce the report with rustup toolchain link, rustup component add rustfmt, and rustup show using the custom toolchain name. Trace how linked toolchains are identified during component installation; done means rustfmt, and subsequently clippy, can be installed or the command gives a clear supported limitation.