Fails to add component on Windows

We're also seeing some breakage for rustup component add --toolchain nightly rustfmt, but only on Windows:

error: invalid value 'C:\Users\runneradmin\.cargo\bin\rustup.exe' for '[+toolchain]': error: "C:\Users\runneradmin\.cargo\bin\rustup.exe" is not a valid subcommand, so it was interpreted as a toolchain name, but it is also invalid. To override the toolchain using the 'rustup +toolchain' syntax, make sure to prefix the toolchain override with a '+'

It's similar, but not the same as marmeladema's error from above.

EDIT: this fixed it:

-          rustup toolchain add nightly --profile minimal
-          rustup component add --toolchain nightly rustfmt
+          rustup toolchain install nightly --profile minimal --component rustfmt

Originally posted by @lnicola in #4220

Start by reviewing the linked GitHub Actions Windows job and the reported rustup component add --toolchain nightly rustfmt command, then compare it with the stated working command using rustup toolchain install. Done means confirming whether the Windows failure still occurs and whether any repository change remains necessary.