unable to re-launch application within gdb

With early injection, re-launching within gdb fails with:

<Stopping application /usr/bin/ls (11814)>
[Inferior 1 (process 11814) exited normally]
(gdb) run
Starting program: /work/dr/git/build_x64_dbg/lib64/debug/libdynamorio.so.5.0 -debug -- ls
<Application  (11936).  Internal Error: DynamoRIO debug check failure: /work/dr/git/src/core/unix/os.c:928 executable_path[0] != '\0' && "i#907: Can't read /proc/self/exe for early injection"

I first observed this on ARM where I debugged by looking at environ, and it seemed like under gdb the new env vars just weren't propagated across the execve.