grep: use posix shebang for egrep/fgrep

Use POSIX shebang for egrep/fgrep instead of the variable/Bash.
The issue affects both master and openwrt-21.02.

Signed-off-by: Vladislav Grigoryev <vg.aetera@gmail.com>

utils/grep/Makefile

@@ -9,7 +9,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=grep
 PKG_VERSION:=3.7
-PKG_RELEASE:=1
+PKG_RELEASE:=2
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.xz
 PKG_SOURCE_URL:=@GNU/grep
@@ -23,6 +23,8 @@ PKG_CPE_ID:=cpe:/a:gnu:grep
 PKG_INSTALL:=1
 PKG_BUILD_PARALLEL:=1
 
+MAKE_FLAGS += SHELL="/bin/sh"
+
 include $(INCLUDE_DIR)/package.mk
 
 define Package/grep