Minor: add comments explaining bad MSRV, output in json

[alamb]

Which issue does this PR close?

Closes #.

Rationale for this change

While debugging #6853 with @andygrove and @viirya , the CI output display for MSRV failures look 🤮 and doesn't tell you what the problem is

https://github.com/apache/arrow-rs/actions/runs/12207850027/job/34060002164?pr=6853

I also tried improving the output display here but it doesn't seem to display the error to logs

• Improve MSRV failure CI JOB output display #6854

What changes are included in this PR?

1. Add comments explaining what to do on failure
2. Change the output to JSON which at least has the actual error in the string, even if it is somewhat hard to see

Here is an example failure message from https://github.com/apache/arrow-rs/actions/runs/12215692321/job/34077953686?pr=6857

{"type":"subcommand_result","subcommand_id":"verify","result":{"toolchain":{"version":"1.70.0","target":"x86_64-unknown-linux-gnu","components":[]},"is_compatible":false,"error":"error: package `clap_lex v0.7.4` cannot be built because it requires rustc 1.74 or newer, while the currently active rustc version is 1.70.0\nEither upgrade to rustc 1.74 or newer, or use\ncargo update -p [email protected] --precise ver\nwhere `ver` is the latest version of `clap_lex` supporting rustc 1.70.0\n"}}

Are there any user-facing changes?

No

[alamb]

Reopened to trigger CI to see if Alexhuszagh/rust-lexical#191 fixes #6858