Always build external example/contract repos with latest stable Rust

The test-with-soroban-examples and test-with-openzeppelin-stellar-contracts
workflows installed Rust with a bare `rustup update` and no toolchain
override, so a rust-toolchain.toml pin (in this repo or the external repo)
could take effect. With the MSRV now at 1.93, an older pinned toolchain
makes cargo's MSRV-aware resolver pick an older soroban-sdk for transitive
deps (e.g. soroban-poseidon) than the path-patched local 26.0.1, compiling
two copies of soroban-sdk and failing with duplicate lang item (panic_impl)
errors. Force RUSTUP_TOOLCHAIN=stable so these builds always use the latest
stable Rust regardless of any pin.

https://claude.ai/code/session_015kBhoioxYh6bD9o6eb9Ksh

Author: leighmcculloch

.github/workflows/test-with-openzeppelin-stellar-contracts.yml

@@ -14,6 +14,14 @@ concurrency:
 # repository does not gain access to anything in this repo.
 permissions: {}
 
+# Always build with the latest stable Rust, overriding any rust-toolchain.toml
+# pinned by this repo or the external contracts repo. A pinned/older toolchain
+# makes cargo's MSRV-aware resolver select an older soroban-sdk for transitive
+# dependencies (e.g. soroban-poseidon) than the path-patched local one, which
+# compiles two copies of soroban-sdk and fails with duplicate lang item errors.
+env:
+  RUSTUP_TOOLCHAIN: stable
+
 jobs:
   collect-crates:
     runs-on: ubuntu-latest

.github/workflows/test-with-soroban-examples.yml

@@ -14,6 +14,14 @@ concurrency:
 # repository does not gain access to anything in this repo.
 permissions: {}
 
+# Always build with the latest stable Rust, overriding any rust-toolchain.toml
+# pinned by this repo or the external examples repo. A pinned/older toolchain
+# makes cargo's MSRV-aware resolver select an older soroban-sdk for transitive
+# dependencies (e.g. soroban-poseidon) than the path-patched local one, which
+# compiles two copies of soroban-sdk and fails with duplicate lang item errors.
+env:
+  RUSTUP_TOOLCHAIN: stable
+
 jobs:
   collect-examples:
     runs-on: ubuntu-latest