build: replace "\\" with "\" in raw string

in Python, a raw string is created using 'r' or 'R' prefix. when
creating the regex using Python string, sometimes, we have to use
"\" to escape the parenthesis so the tools like "sed" can consider
the parenthesis as a capture group. but "\" is also used to escape
strings in Python, in order to put "\" as it is, we use "\" instead
of escaping "\" with "\\" which is obscure.

turns out in
CPython 3.12, it loyally generates "\\" if "\\" is in a raw string.
so "sed" is confused when executing the generated build rule, like:

```
[3/36] ANTLR3 alternator/expressions.g
FAILED: build/debug/gen/alternator/expressionsLexer.cpp build/debug/gen/alternator/expressionsLexer.hpp build/debug/gen/alternator/expressionsParser.cpp build/debug/gen/alternator/expressionsParser.hpp
sed -e '/^#if 0/,/^#endif/d' alternator/expressions.g > build/debug/gen/alternator/expressions.g && antlr3 build/debug/gen/alternator/expressions.g && sed -i -e '/^.*On :.*$/d' build/debug/gen/alternator/expressionsLexer.hpp && sed -i -e '/^.*On :.*$/d' build/debug/gen/alternator/expressionsLexer.cpp && sed -i -e '/^.*On :.*$/d' build/debug/gen/alternator/expressionsParser.hpp && sed -i -e 's/^\\( *\)\\(ImplTraits::CommonTokenType\\* [a-zA-Z0-9_]* = NULL;\)$/\1const \2/' -e '/^.*On :.*$/d' -e '1i using ExceptionBaseType = int;' -e 's/^{/{ ExceptionBaseType\* ex = nullptr;/; s/ExceptionBaseType\* ex = new/ex = new/; s/exceptions::syntax_exception e/exceptions::syntax_exception\& e/' build/debug/gen/alternator/expressionsParser.cpp
sed: -e expression #1, char 80: Unmatched ) or \)
```

in this change, we replace "\\" with "\" in raw string where we
intend to put a single "\" in the created string. this is accepted
by both Python 3.11 shipped along with Fedora 38 and Python 3.12
shipped along with Fedora 39.

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

configure.py

@@ -1956,7 +1956,7 @@ def write_build_file(f,
                      && sed -i -e '/^.*On :.*$$/d' $builddir/{mode}/gen/${{stem}}Lexer.hpp $
                      && sed -i -e '/^.*On :.*$$/d' $builddir/{mode}/gen/${{stem}}Lexer.cpp $
                      && sed -i -e '/^.*On :.*$$/d' $builddir/{mode}/gen/${{stem}}Parser.hpp $
-                     && sed -i -e 's/^\\( *\)\\(ImplTraits::CommonTokenType\\* [a-zA-Z0-9_]* = NULL;\\)$$/\\1const \\2/' $
+                     && sed -i -e 's/^\\( *\)\\(ImplTraits::CommonTokenType\\* [a-zA-Z0-9_]* = NULL;\)$$/\1const \2/' $
                         -e '/^.*On :.*$$/d' $
                         -e '1i using ExceptionBaseType = int;' $
                         -e 's/^{{/{{ ExceptionBaseType\* ex = nullptr;/; $