Fixed KeyError when loading Windows targets over SMB

[Paradoxis]

Change various ["sysvol"] statements to .get methods as to avoid breaking SMB targets with low-privileged credentials

│                                                                                                  │
│ /home/user/.local/lib/python3.11/site-packages/dissect/target/plugins/os/windows/_os.py:82 in    │
│ add_mounts                                                                                       │
│                                                                                                  │
│    79 │   │   │   self.target.log.debug("", exc_info=e)                                          │
│    80 │   │                                                                                      │
│    81 │   │   # Fallback mount the sysvol to C: if we didn't manage to mount it to any other d   │
│ ❱  82 │   │   if operator.countOf(self.target.fs.mounts.values(), self.target.fs.mounts["sysvo   │
│    83 │   │   │   if "c:" not in self.target.fs.mounts:                                          │
│    84 │   │   │   │   self.target.log.debug("Unable to determine drive letter of sysvol, falli   │
│    85 │   │   │   │   self.target.fs.mount("c:", self.target.fs.mounts["sysvol"])                │
│                                                                                                  │
│ ╭─────────────────────────────────────── locals ────────────────────────────────────────╮        │
│ │ self = <dissect.target.plugins.os.windows._os.WindowsPlugin object at 0x7f87c488a3d0> │        │
│ ╰───────────────────────────────────────────────────────────────────────────────────────╯        │
╰──────────────────────────────────────────────────────────────────────────────────────────────────╯
KeyError: 'sysvol'

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 75.01%. Comparing base (b89b12d) to head (51defd2).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #726 +/- ## ======================================= Coverage 75.01% 75.01% ======================================= Files 294 294 Lines 25016 25018 +2 ======================================= + Hits 18766 18768 +2 Misses 6250 6250 ``` | [Flag](https://app.codecov.io/gh/fox-it/dissect.target/pull/726/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=fox-it) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/fox-it/dissect.target/pull/726/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=fox-it) | `75.01% <100.00%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=fox-it#carryforward-flags-in-the-pull-request-comment) to find out more.

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