Why does sh in the build environment ignore SIGINT and SIGQUIT?

[Foo Chuan Wei]

The shell in the environment where packages are built ignores SIGINT and
SIGQUIT. If I add `(invoke "sh" "-c" "trap")` to a custom build phase,
this output is produced during the build:

    trap -- '' INT
    trap -- '' QUIT

Why does the shell in the build environment need to ignore these two
signals?

For context, this affects the build of the smlnj package
(gnu/packages/sml.scm). The resulting executable seems to inherit the
signal dispositions of the shell where the executable is built, with the
result that CTRL-C is ignored at the sml REPL.