'dune exec' behaves differently with DUNE_BUILD_DIR set to absolute path

[andreypopp]

If I try to set DUNE_BUILD_DIR to an absolute path dune exec fails:

% DUNE_BUILD_DIR="$PWD/_custom" dune exec test/test_main.exe
File unavailable: /Users/andreypopp/Workspace/esy-ocaml/repromise/_custom/default/test/test_main.exe

while relative DUNE_BUILD_DIR (or an absence of such) works:

% dune exec test/test_main.exe
Testing library 'repromise'...
......................................................
Ok. 54 tests ran, 0 tests skipped

% DUNE_BUILD_DIR="./_custom" dune exec test/test_main.exe
Testing library 'repromise'...
......................................................
Ok. 54 tests ran, 0 tests skipped

Hitting this with esy on aantron/repromise but the snippets above are replicated with opam.

[andreypopp]

Thank you!