Fix bench_diskless type checking with mypy 2

[glaziermag]

Mypy 2 rejects the format annotation in the diskless-read benchmark because the earlier loop already defines that name. Rename the second variable to fmt and update its three uses, retaining the Literal["NETCDF3_CLASSIC"] annotation required by write_netcdf.

Fixes #1498.

Validation:

• Reproduced both reported errors on current master with mypy 2.3.1.
• With the fix, mypy --no-incremental examples passes all 14 files and mypy --no-incremental test passes all 57 files using the repository's type stubs on Python 3.14.
• The full netcdf-c master build and test workflow passed on this exact commit (52a6a411), including Stubtest.

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[jswhit]

thanks for this fix!