rustc is not building for custom targets

[DeepeshWR]

rustc is not building for custom targets with the current nightly branch. I suspect this is related to the following commit

As mentioned in the commit, I tried passing the -Zunstable-options flag using the following methods:

export RUSTFLAGS="-Zunstable-options"
export RUSTFLAGS_BOOTSTRAP="-Zunstable-options"
config.set("rust", "rustflags", e(["-Zunstable-options",]))

However, none of these approaches worked. The failure occurs during the stage1 build. My understanding
is that the stage1 build may be stripping or ignoring these flags.

Am I missing something here, or is there a different way to ensure -Zunstable-options is honoured during
the stage1 build?

The error I see is:

Created at: src/bootstrap/src/core/builder/mod.rs:1403:23
Executed at: src/bootstrap/src/core/build_steps/compile.rs:1143:25

--- STDERR vvv
error: error loading target specification: custom targets are unstable and require `-Zunstable-options`
  |
  = help: run `rustc --print target-list` for a list of built-in targets

So, once this commit landed in stable releases, how are custom targets expected to be built using the stable channel? Are there any alternative mechanisms or future plans to support this use case?
The full log is attached below for reference.

rust.log

[bjorn3]

It is now necessary to pass -Z json-target-spec to cargo for custom targets to work. RUSTFLAGS doesn't work as it is ignored for the rustc invocation that cargo uses to learn basic information about a target like the file extensions and supported crate types.