scripts/gen-btf.sh: Use CONFIG_SHELL for execution

scripts/gen-btf.sh: Use CONFIG_SHELL for execution

According to the docs [1], kernel build scripts should be executed via
CONFIG_SHELL, which is sh by default.

Fixup gen-btf.sh to be runnable with sh, and use CONFIG_SHELL at every
invocation site.

See relevant discussion for context [2].

[1] https://docs.kernel.org/kbuild/makefiles.html#script-invocation
[2] https://lore.kernel.org/bpf/CAADnVQ+dxmSNoJAGb6xV89ffUCKXe5CJXovXZt22nv5iYFV5mw@mail.gmail.com/

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Tested-by: Gary Guo <gary@garyguo.net>
Reported-by: Gary Guo <gary@garyguo.net>
Suggested-by: Thomas Weißschuh <linux@weissschuh.net>
Fixes: 522397d05e7d ("resolve_btfids: Change in-place update with raw binary output")
Link: https://lore.kernel.org/r/20260121181617.820300-1-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>