Update wasi-sdk used in testing

(backport <#4747>)
(cherry picked from commit 3fb070e)

Author: alexcrichton

ci/wasi.sh

@@ -12,7 +12,7 @@ apt-get install -y --no-install-recommends \
 # Wasmtime is used to execute tests and wasi-sdk is used to compile tests.
 # Download appropriate versions here and configure various flags below.
 wasmtime=35.0.0
-wasi_sdk=25
+wasi_sdk=27
 
 curl -L https://github.com/bytecodealliance/wasmtime/releases/download/v$wasmtime/wasmtime-v$wasmtime-x86_64-linux.tar.xz |
     tar xJf -