bug#48018: ImageMagick’s ‘convert’ fails to delegate to ‘dwebp’

["Ludovic Courtès" <ludo@gnu.org>]

ImageMagick’s ‘convert’ refers to the ‘dwebp’ program with a leading
space, which prevents conversion to/from webp:

--8<---------------cut here---------------start------------->8---
$ guix time-machine --commit=2a624253bc55e2a94f6581d6e790303575436c96 -- \
   environment -C --ad-hoc strace imagemagick libwebp -- \
   strace -f -e execve convert t.webp t.png
execve("/gnu/store/2rmnafvwqvl460qa370gn18yl09dx91h-profile/bin/convert", ["convert", "t.webp", "t.png"], 0x7ffe6637b988 /* 10 vars */) = 0
strace: Process 5 attached
[pid     5] execve("/gnu/store/2rmnafvwqvl460qa370gn18yl09dx91h-profile/bin/ dwebp", [" dwebp", "-pam", "/tmp/magick-FacyU5v0_s0CnpxpMzs0"..., "-o", "/tmp/magick-4R_OusxZBx5fjGrfq0KU"...], 0x7ffd7f3538e8 /* 10 vars */) = -1 ENOENT (No such file or directory)
[pid     5] +++ exited with 1 +++
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=5, si_uid=1000, si_status=1, si_utime=0, si_stime=0} ---
convert: unable to open image `t.webp': No such file or directory @ error/blob.c/OpenBlob/2924.
convert: delegate failed `' dwebp' -pam '%i' -o '%o'' @ error/delegate.c/InvokeDelegate/1970.
convert: unable to open file `/tmp/magick-e8GzrEcauG0mZICNZggy_vG77mtn29lu': No such file or directory @ error/constitute.c/ReadImage/615.
convert: no images defined `t.png' @ error/convert.c/ConvertImageCommand/3229.
+++ exited with 1 +++
--8<---------------cut here---------------end--------------->8---

Ludo’.