abuild-fetch clobbers download in progress

Regularly on the armv7/armhf builders, builds fail because abuild-fetch clobbers an existing download in progress.

The builders share the same distfiles, so both builders will try to download the source and store it at the same location. For example:

nodejs on armhf

>>> nodejs: Fetching https://nodejs.org/dist/v12.21.0/node-v12.21.0.tar.gz
Partial download found. Trying to resume.
** Resuming transfer from byte position 25541336
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
24 26.4M   24 6526k    0     0  18.3M      0  0:00:01 --:--:--  0:00:01 18.3M
100 26.4M  100 26.4M    0     0  27.9M      0 --:--:-- --:--:-- --:--:-- 27.9M
>>> nodejs: Fetching https://distfiles.alpinelinux.org/distfiles/v3.12/node-v12.21.0.tar.gz
>>> nodejs: Checking sha512sums...
node-v12.21.0.tar.gz: FAILED
sha512sum: WARNING: 1 of 1 computed checksums did NOT match

nodejs on armv7

>>> nodejs: Fetching https://nodejs.org/dist/v12.21.0/node-v12.21.0.tar.gz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
100 50.7M  100 50.7M    0     0  80.1M      0 --:--:-- --:--:-- --:--:-- 80.1M
>>> nodejs: Fetching https://distfiles.alpinelinux.org/distfiles/v3.12/node-v12.21.0.tar.gz
>>> nodejs: Checking sha512sums...
node-v12.21.0.tar.gz: FAILED
sha512sum: WARNING: 1 of 1 computed checksums did NOT match

Retrying the build fixes it.