vamos: create shared volume dirs with exist_ok

[codewiz]

When many vamos instances run in parallel against a shared HOME (the gcc testsuite launches dozens at once), the first burst races to create ~/.vamos/volumes, the auto system: volume, and its auto-assign subdirs. os.makedirs() without exist_ok raises FileExistsError for all but the winner, so those instances abort with:

path: ERROR: error creating volume base dir: .../.vamos/volumes -> [Errno 17] File exists
path: ERROR: invalid volume spec: 'system:?create'

or can't create relative dir: c. In the AmigaPorts m68k-amigaos-gcc CI this shows up as a handful of spurious -O0 execution FAILs per run, on whichever tests happen to be in the first concurrent batch. The programs never touch the filesystem; the system: volume is just vamos's default env (a sys: assign for the CWD), so sharing it read-only is fine, only its creation races.

This mirrors PR #3, which fixed the same class of race for temp (ram:) volumes via mkdtemp. Here the three os.makedirs that build the shared system: dirs get exist_ok=True.

Tested: 960 concurrent launches sharing one fresh HOME, 0 failures (was ~1-2% before). pytest test/unit -k "path or volume" -> 70 passed.

[codewiz]

Validated in the toolchain CI (release-gate config: Linux build + gcc testsuite under vamos, run_testsuite=true, pointed at this branch via amitools_git): https://github.com/AmigaPorts/m68k-amigaos-gcc/actions/runs/33063607276

The three spurious -O0 execution FAILs from the volume race are gone: expected passes 24408 -> 24411 (exactly the three flaky tests), unexpected failures 18 -> 15. The remaining 15 are pre-existing real issues unrelated to vamos (conversion.c __floatundixf, pr78622 %hhd, pr39228).