Fix test compile: String.write is no longer a static method

[conorbronsdon]

The test workflow has been red on main since a Mojo nightly bump between 7/29 and 7/31. No source change caused it — the stdlib made String.write an instance method, so both overloads now take mut self and the old static call no longer resolves:

test/test_errors.mojo:15:18: error: no matching function in call to 'write'
note: candidate not viable: value passed to mutable argument 'self' must be mutable
note: candidate not viable: missing required argument: 'value'

String.__init__ takes a variadic pack of Writable args (string.mojo:456) and Error conforms to Writable (error.mojo:138), so String(e) is the direct replacement and writes the same bytes.

Test-only; nothing in src/ used the static form.

Root cause worth noting separately: test.yml installs mojo from the nightly index with --prerelease allow and no version pin, so every run resolves whatever nightly is current. That's why a frozen repo's CI went green → red with no commits in between. Fixing the call site clears today's breakage but not the pattern.

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