fix(DownloadService): use Path.Combine for combining paths

[GerardSmit]

Before:

Created temporary directory C:\Users\x\AppData\Local\Temp\/MangaMagnet-a34bc563-9467-4944-969b-0e9b2f4f2e7a

After:

Deleted temporary directory C:\Users\x\AppData\Local\Temp\MangaMagnet-2eabd02e-93b6-4bb9-87d8-aeba77ac1c7f

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +31.52%	✅ 0.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (2e5b719)	1268	50	3.94%
Head commit (5ec0c30)	2600 (+1332)	922 (+872)	35.46% (+31.52%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#9)	1	0	0.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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