fix(files): Writing new files works across devices

[alexpovel]

persist of tempfile does not work across
device boundaries, so if the device backing the
temporary directory is e.g. tmpfs, then
persist fails. It actually had no useful
benefit, so we can just remove it and write to the destination directly.

Reproduced bug and confirmed fix on a Debian
machine with:

$ sudo mkdir -p /path/to/temp_dir
$ sudo mount -t tmpfs -o size=100m tmpfs /path/to/temp_dir
$ sudo mount | rg 'temp'
tmpfs on /path/to/temp_dir type tmpfs (rw,relatime,size=102400k,inode64)
$ export TMPDIR=/path/to/temp_dir
$ cargo run -- --python class a whatever

TMPDIR env var is respected by tempfile. The
above cargo run fails without this patch. (There is an end-to-end test for file writing, but that
has never failed for some reason... seems like
none of my machines (including Ubuntu 24) or CI
exhibited the issue).

Closes #145

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.69%. Comparing base (3b8abfa) to head (b238e31).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #148      +/-   ##
==========================================
+ Coverage   86.58%   86.69%   +0.11%     
==========================================
  Files          32       32              
  Lines        1975     1969       -6     
==========================================
- Hits         1710     1707       -3     
+ Misses        265      262       -3     

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[alexpovel]

Very hard to reproduce. Even cargo test with a modified TMPDIR doesn't reproduce it. There are tests for it:

srgn/tests/cli.rs

Line 399 in 3b8abfa

fn test_cli_files(

and I can confirm they also respect and use TMPDIR (export TMPDIR=...), reading and writing to a directory on a different device (tmpfs), but not triggering the failure. Only manual cargo run does. This suggests the tests are faulty but wasn't able to track that down.