[PR gdb/28369] Use get_shell on gdb/ser-pipe.c

PR gdb/28369 reports that gdb/ser-pipe.c has an 'execl' function call
with a hard-coded "/bin/sh" as its argument.  We've had 'get_shell'
for a while now, which is conscious about the SHELL environment and a
better alternative to always calling "/bin/sh".

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28369