Cannot continue over exec

When running a shell script which execs a binary, nothing gets printed on stdout with rr replay.

Environment

Tested on an Haswell laptop, on master and the last release

Reproducer

cat >hello-world.c <<EOF
#include <stdio.h>

int main() {
  puts("hello world");
  return 0;
}
EOF
make hello-world

cat >wrapper.sh <<EOF
#!/bin/sh
exec "./hello-world"
EOF
chmod +x wrapper.sh

rr record ./hello-world
rr replay
# hello world gets printed to stdout
rr record ./wrapper.sh
rr replay
[SNIP]
(rr) continue
# nothing

Start by running the provided hello-world.c and wrapper.sh reproducer with rr record and rr replay, then inspect the replay path reached by the shell script's exec. The fix is complete when rr replay prints "hello world" after continuing through the wrapper, matching direct execution.