Make `rustfmt.toml` a symlink

Instead of making a copy of `soong/scripts/rustfmt.toml`, we now
reference it via a symlink. The only change compared with the existing
file is that the edition changes from 2018 to 2021. I ran `rustfmt` on
all files and there were no formatting changes because of this.

Bug: b/229825251
Test: manual
Change-Id: Ia0e833e6bcd97da01715a964b15255014b11ef6f
2 files changed
tree: 0e55919142d1d77e4b63ee7c13f4f6d0ffb5f06b
  1. conf/
  2. src/
  3. utils/
  4. .clang-format
  5. Android.bp
  6. CleanSpec.mk
  7. NOTICE
  8. OWNERS
  9. PREUPLOAD.cfg
  10. run_unit_tests.sh