openbsd.yml: Replace cargo cache related disk space hack

[oech3]

Avoid sacrificing build performance.

[oech3]

2025-12-30T09:06:24.3648012Z Filesystem     Size    Used   Avail Capacity  Mounted on
2025-12-30T09:06:24.3649170Z /dev/sd0a      986M   96.4M    840M    11%    /
2025-12-30T09:06:24.3650248Z /dev/sd0k      117G   19.2M    111G     1%    /home
2025-12-30T09:06:24.3650755Z /dev/sd0d      3.9G   10.0K    3.7G     1%    /tmp
2025-12-30T09:06:24.3651220Z /dev/sd0f     19.7G    1.7G   17.0G     9%    /usr
2025-12-30T09:06:24.3651714Z /dev/sd0g      986M    344M    592M    37%    /usr/X11R6
2025-12-30T09:06:24.3652214Z /dev/sd0h     19.4G    169M   18.2G     1%    /usr/local
2025-12-30T09:06:24.3652701Z /dev/sd0j      5.8G    2.0K    5.5G     1%    /usr/obj
2025-12-30T09:06:24.3653172Z /dev/sd0i      4.8G    2.0K    4.6G     1%    /usr/src
2025-12-30T09:06:24.3653669Z /dev/sd0e     15.5G   20.3M   14.7G     1%    /var
->
2025-12-30T09:06:42.5320849Z Filesystem     Size    Used   Avail Capacity  Mounted on
2025-12-30T09:06:42.5322012Z /dev/sd0a      986M   96.5M    840M    11%    /
2025-12-30T09:06:42.5322566Z /dev/sd0k      117G   19.2M    111G     1%    /home
2025-12-30T09:06:42.5323271Z /dev/sd0d      3.9G   10.0K    3.7G     1%    /tmp
2025-12-30T09:06:42.5323903Z /dev/sd0f     19.7G    739M   18.0G     4%    /usr
2025-12-30T09:06:42.5324498Z /dev/sd0g      986M    2.0K    937M     1%    /usr/X11R6
2025-12-30T09:06:42.5325058Z /dev/sd0h     19.4G    621M   17.8G     4%    /usr/local
2025-12-30T09:06:42.5325603Z /dev/sd0j      5.8G    2.0K    5.5G     1%    /usr/obj
2025-12-30T09:06:42.5326200Z /dev/sd0i      4.8G    2.0K    4.6G     1%    /usr/src
2025-12-30T09:06:42.5326799Z /dev/sd0e     15.5G   20.4M   14.7G     1%    /var