[buck2] add platform-independent buck2 installation instructions (#402)

Author: lugehorsam

infra/setup-buck2.sh

@@ -10,3 +10,10 @@ unzstd buck2-x86_64-unknown-linux-musl.zst
 # Change its name and make it executable.
 mv buck2-x86_64-unknown-linux-musl buck2-exe
 chmod +x buck2-exe
+
+# Alternatively, for a platform-independent method that requires building from source:
+# Install the specific nightly version
+# rustup install nightly-2025-02-16
+
+# Use that specific version to install Buck2
+# cargo +nightly-2025-02-16 install --git https://github.com/facebook/buck2 --rev 201beb86106fecdc84e30260b0f1abb5bf576988 buck2