Update Rust toolchain to 1.86 with backward compatibility (#11)

Signed-off-by: Simon Davies <simongdavies@users.noreply.github.com>
Signed-off-by: Mark Rossetti <marosset@microsoft.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

Author: Copilot

action.yml

@@ -5,9 +5,9 @@ description: "Common setup steps for GitHub workflows in the Hyperlight project"
 
 inputs:
   rust-toolchain:
-    description: "(Default: 1.85.0) Rust toolchain specification to install - see https://rust-lang.github.io/rustup/concepts/toolchains.html#toolchain-specification"
+    description: "(Default: 1.86.0) Rust toolchain specification to install - see https://rust-lang.github.io/rustup/concepts/toolchains.html#toolchain-specification"
     required: false
-    default: "1.85.0"
+    default: "1.86.0"
 
 runs:
   using: composite
@@ -148,6 +148,7 @@ runs:
       if: ${{ (runner.os == 'Linux') }}
       run: |
         rustup toolchain install 1.81.0
+        rustup toolchain install 1.85.0
       shell: bash
 
     - name: Set up env vars (Linux)
@@ -189,6 +190,7 @@ runs:
       if: ${{ (runner.os == 'Windows') }}
       run: |
         rustup toolchain install 1.81.0
+        rustup toolchain install 1.85.0
       shell: pwsh
 
     - name: Set up env vars (Windows)