bug#33781: tramp tests abort on hydra

[Glenn Morris]

I installed nix locally and reproduced the issue.
AFAICS it is due to

(setq attr (directory-files-and-attributes tmp-name2))

throwing an error with

tmp-name2 =
#("/mock:AAAA.BBBBBBBB.CCC:/AAAA_data1/scratch/UUUUUUU/software64/nix/tmp/tramp-testqSsjHd/bla"
 6 23 (tramp-default t))

AFAICS from the tramp debug buffer, the problem is a failure to find
"sed" on the remote system. It should be using
/nix/store/ny5p32137wfyzdm485xfdck21w1gyl3g-gnused-4.5/bin/sed, in much
the same way as it using
/nix/store/wm8va53fh5158ipi0ic9gir64hrvqv1z-coreutils-8.29/bin/ls .

;; Emacs: 27.0.50 Tramp: 2.4.1-pre -*- mode: outline; -*-
16:53:21.240431 tramp-send-command (6) # test -d 
/AAAA_data1/scratch/UUUUUUU/software64/nix/tmp/tramp-testRd0FTK/bla 
2>/dev/null; echo tramp_exit_status $?
16:53:21.241805 tramp-wait-for-regexp (6) # 
tramp_exit_status 0
///4edae1a3102909a43b8d4c6e6e62ec10#$
16:53:21.248310 tramp-do-directory-files-and-attributes-with-stat (5) # 
directory-files-and-attributes with stat: 
/AAAA_data1/scratch/UUUUUUU/software64/nix/tmp/tramp-testRd0FTK/bla
16:53:21.249681 tramp-send-command (6) # cd 
/AAAA_data1/scratch/UUUUUUU/software64/nix/tmp/tramp-testRd0FTK/bla && echo 
"("; (/nix/store/wm8va53fh5158ipi0ic9gir64hrvqv1z-coreutils-8.29/bin/ls 
--color=never --quoting-style=literal --show-control-chars -a | xargs env 
QUOTING_STYLE=locale 
/nix/store/wm8va53fh5158ipi0ic9gir64hrvqv1z-coreutils-8.29/bin/stat -c 
'(/////%n///// (/////%N/////) %h %u %g %X %Y %Z %s /////%A///// t %i -1)' -- 
2>/dev/null | sed -e 's/"/\\"/g' -e 's/\/\/\/\/\//"/g'); echo ")" 2>/dev/null; 
echo tramp_exit_status $?
16:53:21.257525 tramp-wait-for-regexp (6) # 
(
sh: sed: command not found
)
tramp_exit_status 0
///4edae1a3102909a43b8d4c6e6e62ec10#$
16:53:21.279389 tramp-send-command (6) # cd / && rm -rf 
/AAAA_data1/scratch/UUUUUUU/software64/nix/tmp/tramp-testRd0FTK 2>/dev/null; 
echo tramp_exit_status $?
16:53:21.286694 tramp-wait-for-regexp (6) # 
tramp_exit_status 0
///4edae1a3102909a43b8d4c6e6e62ec10#$