scripts/coverage.sh: use POSIX command instead of which (#2637)

Debian deprecated `which` in `debianutils` in favor of `command`.

`which` outputs this to stderr now:
/usr/bin/which: this version of `which' is deprecated; use `command -v' in scripts instead.

scripts/coverage.sh

@@ -1,7 +1,7 @@
 #!/bin/sh
 set -eu
 
-if ! which grcov 2>/dev/null 1>&2; then
+if ! command -v grcov >/dev/null; then
     echo >&2 '`grcov` not found. Check README at https://github.com/mozilla/grcov for setup instructions.'
     echo >&2 'Run `cargo install grcov` to build `grcov` from source.'
     exit 1