fixup! avm: force file sync (#1457)

[steviez]

Sorry for second PR; didn't browse deep enough to see this one could be problematic as well. Don't see any other instances of write_all() in this file's tests.

     Running unittests (target/debug/deps/avm-57a23adf9ea2b364)
 
running 7 tests
test tests::test_version_binary_path ... ok
test tests::test_current_version_file_path ... ok
test tests::test_uninstall_non_installed_version - should panic ... ok
test tests::test_ensure_paths ... ok
test tests::test_read_installed_versions ... ok
test tests::test_uninstalled_in_use_version - should panic ... FAILED
test tests::test_current_version ... ok
 
failures:
 
---- tests::test_uninstalled_in_use_version stdout ----
thread 'tests::test_uninstalled_in_use_version' panicked at 'called `Result::unwrap()` on an `Err` value: Could not parse version file: unexpected end of input while parsing major version number', avm/src/lib.rs:121:38
note: panic did not contain expected string
      panic message: `"called `Result::unwrap()` on an `Err` value: Could not parse version file: unexpected end of input while parsing major version number"`,
 expected substring: `"anchor-cli 0.18.2 is currently in use"`
 
failures:
    tests::test_uninstalled_in_use_version