Fix the default value of temp-dir in --help

[gargnitingoogle]

Description

Fix the default value of temp-dir in --help

Show the default value of temp-dir, which is "/tmp" in code, rather than keeping it ambiguous.

Link to the issue in case of a bug fix.

b/397660108

Testing details

1. Manual

   $ git switch master && go run . --help | grep "\/tmp"
         --temp-dir string                              Path to the temporary directory where writes are staged prior to upload to Cloud Storage. (default: system default, likely /tmp)
   $ git switch - && go run . --help | grep "\/tmp"
         --temp-dir string                              Path to the temporary directory where writes are staged prior to upload to Cloud Storage. (default: "/tmp")

2. Unit tests - NA
3. Integration tests - NA

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.45%. Comparing base (260e33c) to head (d0b16cf).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3044   +/-   ##
=======================================
  Coverage   75.45%   75.45%           
=======================================
  Files         126      126           
  Lines       17724    17724           
=======================================
  Hits        13374    13374           
  Misses       3792     3792           
  Partials      558      558           

Flag	Coverage Δ
unittests	75.45% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[kislaykishore]

Thanks for the link, @gargnitingoogle. We also use tempdir to create anonymous files (link). That tempdir is resolved as per this that uses the TMPDIR environment variable if available and otherwise falls back to /tmp.
So, the ideal fixes here would be to align on using the same tempdir everywhere, infer it programmatically and then show it in the usage doc.