build: Replace egrep with grep -E
Avoid a warning at build time:
egrep: warning: egrep is obsolescent; using grep -E
The egrep command has been obsolete since 2007; starting with GNU Grep 3.8, calling egrep and fgrep will print out a warning.
For more information, see: https://lists.gnu.org/archive/html/info-gnu/2022-09/msg00001.html