gh-35565: replace obsolescent egrep

    
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### 📚 Description

replace the last 2 uses of `egrep` in our source code, using the
replacement `grep -E`

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URL: #35565
Reported by: Frédéric Chapoton
Reviewer(s): Matthias Köppe

src/bin/sage-grep

@@ -2,4 +2,4 @@
 
 cd "$SAGE_SRC"
 
-find sage -print | GREP_OPTIONS= egrep '.py([xdi])?$' | xargs grep "$@"
+find sage -name "*.py" -o -name "*.pyx" -o -name "*.pxi" -o -name "*.pxd" | xargs grep "$@"

src/bin/sage-grepdoc

@@ -2,4 +2,4 @@
 
 cd "$SAGE_DOC"
 
-find html -print | GREP_OPTIONS= egrep '.html$' | xargs grep "$@"
+find html -name '*.html' | xargs grep "$@"