rust: build correctly without GNU sed

From e509b5b8be (rust: support for Windows, 2025-10-15), we check cargo's information to decide which library to build. However, that check mistakenly used "sed -s" ("consider files as separate rather than as a single, continuous long stream"), which is a GNU extension. The build thus fails on macOS with "meson -Drust=enabled", which comes with BSD-derived sed. Instead, use the intended "sed -n" and print the matching section of the output. This failure mode likely went unnoticed on systems with GNU sed (common for developer machines and CI) because, in those instances, the output being matched by case is the full cargo output (which either contains the string "-windows-" or doesn't). Helped-by: Eric Sunshine <sunshine@sunshineco.com> Helped-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>