misc/claude-code: Skip elftoolchain strip on Linux binaries

install(1) with -s runs FreeBSD's strip on the prebuilt Linux claude
binary, which fails with "elf_update() failed: Layout constraint
violation" and breaks the stage phase. Set STRIP= to install the
binary verbatim (stripping a foreign Linux ELF is pointless anyway).

Reviewed by: yuri
Differential Revision: https://reviews.freebsd.org/D58338