Fix temp dir permission problem on multi-user machines

[stefanhaller]

In 3a9dbf7 we created a global /tmp/lazygit/ folder that contains the temp directories of each running instance, to avoid polluting /tmp with multiple folders. The problem with that approach was that the folder was created with 700 permissions, so if multiple users were using lazygit on the same machine (e.g. a server), all users except the first one would get fatal errors on startup.

Fix this by creating temp folders containing the user's uid.

Fixes #4793.

[codacy-production]

Coverage summary from Codacy

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Coverage variation	Diff coverage
Report missing for bcb95bd1	✅ 69.23%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (bcb95bd)	Report Missing	Report Missing	Report Missing
Head commit (71813fd)	57213	49720	86.90%

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 (#4796)	13	9	69.23%

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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