Ignore buggy clippy::duplicated_attributes lint

rust-lang/rust-clippy#12537
rust-lang/rust-clippy#12538

```
error: duplicated attribute
     --> src\imp\mod.rs:168:21
      |
  168 |             any(not(portable_atomic_no_asm), portable_atomic_unstable_asm),
      |                     ^^^^^^^^^^^^^^^^^^^^^^
      |
  note: first defined here
     --> src\imp\mod.rs:164:21
      |
  164 |             any(not(portable_atomic_no_asm), portable_atomic_unstable_asm),
      |                     ^^^^^^^^^^^^^^^^^^^^^^
  help: remove this attribute
     --> src\imp\mod.rs:168:21
      |
  168 |             any(not(portable_atomic_no_asm), portable_atomic_unstable_asm),
      |                     ^^^^^^^^^^^^^^^^^^^^^^
      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#duplicated_attributes
      = note: `-D clippy::duplicated-attributes` implied by `-D warnings`
      = help: to override `-D warnings` add `#[allow(clippy::duplicated_attributes)]`

  error: duplicated attribute
     --> src\imp\mod.rs:168:46
      |
  168 |             any(not(portable_atomic_no_asm), portable_atomic_unstable_asm),
      |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      |
  note: first defined here
     --> src\imp\mod.rs:164:46
      |
  164 |             any(not(portable_atomic_no_asm), portable_atomic_unstable_asm),
      |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  help: remove this attribute
     --> src\imp\mod.rs:168:46
      |
  168 |             any(not(portable_atomic_no_asm), portable_atomic_unstable_asm),
      |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#duplicated_attributes

  error: duplicated attribute
     --> src\imp\mod.rs:179:36
      |
  179 |         all(target_arch = "s390x", portable_atomic_unstable_asm_experimental_arch),
      |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      |
  note: first defined here
     --> src\imp\mod.rs:173:13
      |
  173 |             portable_atomic_unstable_asm_experimental_arch,
      |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  help: remove this attribute
     --> src\imp\mod.rs:179:36
      |
  179 |         all(target_arch = "s390x", portable_atomic_unstable_asm_experimental_arch),
      |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#duplicated_attributes

  error: duplicated attribute
     --> src\imp\mod.rs:205:5
      |
  205 |     target_arch = "avr",
      |     ^^^^^^^^^^^^^^^^^^^
      |
  note: first defined here
     --> src\imp\mod.rs:195:5
      |
  195 |     target_arch = "avr",
      |     ^^^^^^^^^^^^^^^^^^^
  help: remove this attribute
     --> src\imp\mod.rs:205:5
      |
  205 |     target_arch = "avr",
      |     ^^^^^^^^^^^^^^^^^^^
      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#duplicated_attributes

  error: duplicated attribute
     --> src\imp\mod.rs:206:5
      |
  206 |     target_arch = "msp430",
      |     ^^^^^^^^^^^^^^^^^^^^^^
      |
  note: first defined here
     --> src\imp\mod.rs:196:5
      |
  196 |     target_arch = "msp430",
      |     ^^^^^^^^^^^^^^^^^^^^^^
  help: remove this attribute
     --> src\imp\mod.rs:206:5
      |
  206 |     target_arch = "msp430",
      |     ^^^^^^^^^^^^^^^^^^^^^^
      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#duplicated_attributes

  error: duplicated attribute
     --> src\imp\mod.rs:210:5
      |
  210 |     feature = "critical-section",
      |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      |
  note: first defined here
     --> src\imp\mod.rs:194:5
      |
  194 |     feature = "critical-section",
      |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  help: remove this attribute
     --> src\imp\mod.rs:210:5
      |
  210 |     feature = "critical-section",
      |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#duplicated_attributes
```

.github/workflows/ci.yml

@@ -21,7 +21,10 @@ env:
   RUST_BACKTRACE: 1
   RUST_TEST_THREADS: 1
   RUSTDOCFLAGS: -D warnings
-  RUSTFLAGS: -D warnings
+  # This lint is buggy and also crate-level #![allow(..)] doesn't work.
+  # https://github.com/rust-lang/rust-clippy/issues/12537
+  # https://github.com/rust-lang/rust-clippy/issues/12538
+  RUSTFLAGS: -D warnings -A unknown_lints -A clippy::duplicated_attributes
   RUSTUP_MAX_RETRIES: 10
   # NB: sync with:
   # - docs.rs metadata in Cargo.toml