shell: single-quote the profile line before echoing it

[rawsun007]

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Follow-up to #24035, which named this as the part it deliberately left alone.

set_variable_in_profile and prepend_path_in_profile build a command for the user to paste, wrapping the whole profile line in literal single quotes. A single quote in the value closes that string early:

$ brew ruby -e 'require "utils/shell"; ENV["SHELL"]="/bin/bash"; puts Utils::Shell.prepend_path_in_profile("/Users/o'brien/bin")'
echo 'export PATH=/Users/o\'brien/bin:$PATH' >> /Users/me/.bash_profile

$ bash --norc -c "echo 'export PATH=/Users/o\'brien/bin:\$PATH' >> prof"
bash: -c: line 0: unexpected EOF while looking for matching `''

Nothing is written. The \ that sh_quote adds is literal inside single quotes, so it does not escape anything; a single-quoted string can only be interrupted, escaped and resumed.

The pwsh branch already passes its whole line through pwsh_quote. sh_single_quote does the same for the Bourne, rc, csh and fish branches.

Emitting the command for five values (apostrophe, space, $, double quote, plain) under bash, zsh, sh, ksh and tcsh, then running it and sourcing the profile it writes: before, the five apostrophe cases fail to run at all, one per shell; after, all 30 run and source back the exact value. The output is byte-identical to today's for every value that does not contain a quote, so nothing that works now changes.

Reachable wherever HOMEBREW_PREFIX or a keg path contains an apostrophe, which on Linux means a home-directory install for someone whose username has one. The callers are brew link for keg-only formulae, the keg-only caveats, and the HOMEBREW_TEMP advice in the Linux diagnostic.

Three new examples, one per changed behaviour; reverting only utils/shell.rb fails exactly those three and nothing else. brew tests also passes for caveats, diagnostic, cmd/link and cmd/--env.

rc is covered by the same change but I have no rc to run it under, so that branch is reasoned from the shared echo '...' shape rather than measured. Saying so rather than implying I tested it.

AI disclosure: written with Claude Code (Claude Opus 5) on my account. The before/after counts come from a harness that ran each emitted command in the real shell and sourced the profile it produced. I answer review comments myself.