update channel (#1420)

Summary:

- `fbgs 'nightly-2025-06-20' -l | cut -d / -f 2- | xargs sed -i -e 's/nightly-2025-06-20/nightly-2025-08-01/'`

- updated `fbcode/buck2/rust-toolchain` to remove nonexisting files from the instructions

- downloaded the nightly toolchain to get the commit hash
  - `rustc +nightly-2025-08-01 --version`

Differential Revision: D83865193

Author: cjlongoria

rust-toolchain

@@ -1,3 +1,3 @@
-# @rustc_version: rustc 1.89.0-nightly (255aa2208 2025-06-19)
+# @rustc_version: rustc 1.90.0-nightly (adcb3d3b4 2025-07-31)
 [toolchain]
-channel = "nightly-2025-06-20"
+channel = "nightly-2025-08-01"