Auto merge of #46207 - kennytm:kill-grep, r=alexcrichton

Auto merge of #46207 - kennytm:kill-grep, r=alexcrichton

Replace most call to grep in run-make by a script that cat the input.

Introduced a new `src/etc/cat-and-grep.sh` script (called in run-make as `$(CGREP)`), which prints the input and do a grep simultaneously. This is mainly used to debug spurious failures in run-make, such as the spurious error in #45810, as well as real errors such as #46126.

(cc #40713)

Some `grep` still remains, mainly the `grep -c` calls that count the number of matches and print the result to stdout.