Suppress use of current catcodes in control sequence in regex replacements

[blefloch]

Recent changes to catcode handling in l3regex break the following document, as reported by @u-fischer at #621.

\documentclass{article}

\ExplSyntaxOn
\cs_new_protected:Npn \test #1
  {
    \tl_set:Nn \l_tmpa_tl { #1 }
    \regex_replace_all:nnN
     { (a) } 
     { \c{tl_show:n} } %fails because of the : in the command name)
     \l_tmpa_tl
  }
\ExplSyntaxOff

\begin{document}
\catcode`\:=13 \def:{x}
xx
\test{a}

\end{document}