idtools: avoid direct use of C.stderr to fix musl cgo build failures

[clan]

On musl-based systems, stderr is declared as FILE *const.

Referencing stderr directly from Go code (via C.stderr) causes cgo to generate assignment code for a const-qualified pointer, which is invalid C and fails to compile.

Both gcc and clang reject the generated code with error messages below:

clang:

cgo-gcc-prolog:85:9: error: cannot assign to variable '_cgo_r' with const-qualified type 'typeof (_cgo_a->r)' (aka 'struct _IO_FILE *const')
cgo-gcc-prolog:83:24: note: variable '_cgo_r' declared const here
cgo-gcc-prolog:88:12: error: cannot assign to non-static data member 'r' with const-qualified type 'FILE *const' (aka 'struct _IO_FILE *const')
cgo-gcc-prolog:80:15: note: non-static data member 'r' declared const here

gcc:

cgo-gcc-prolog:85:9: error: assignment of read-only variable '_cgo_r'
cgo-gcc-prolog:88:12: error: assignment of read-only member 'r'

This patch avoids referencing C.stderr from Go code and instead returns stderr from a small C helper function. This keeps the usage entirely in C and avoids cgo’s broken handling for const-qualified global objects.

Checklist

Ensure you have completed the following checklist for your pull request to be reviewed:

• Certify you wrote the patch or otherwise have the right to pass it on as an open-source patch by signing all
  commits. (git commit -s). (If needed, use git commit -s --amend). The author email must match
  the sign-off email address. See CONTRIBUTING.md
  for more information.
• Referenced issues using Fixes: #00000 in commit message (if applicable)
• Tests have been added/updated (or no tests are needed)
• Documentation has been updated (or no documentation changes are needed)
• All commits pass make validatepr (format/lint checks)
• Release note entered in the section below (or None if no user-facing changes)

Does this PR introduce a user-facing change?

None