Fix build.py: escape compiler stderr before passing to rich Panel

Compiler error messages (e.g. from gfortran LTO) can contain paths
like [/path/to/file.mk:62: file.ltrans.o] which rich interprets as
markup closing tags, crashing with MarkupError. Wrap with Text() to
treat content as plain text, preventing the crash.

Replaces malformed previous commit.

Author: sbryngelson