fix: include bound glibc in compiler environment

[ZheFeng7110]

Summary

• add the exact glibc@<version> payload library directory to Linux compiler child environments
• keep runtime selection explicit; do not scan installed glibc versions
• cover the initial GCC --version probe failing with exit 127 when the directory is absent

Closes #484

Test plan

• git diff --check
• managed GCC 16.1.0 --version succeeds with the bound private glibc plus gcc-runtime directories
• managed GCC 16.1.0 compiles a C++23 source with the bound private glibc, gcc-runtime, and managed binutils
• mcpp test (full self-host build was terminated by an OOM kill while compiling cli.m.cppm)
• CI