Update wasi-sdk in CI, plus some script updates

* Update to wasi-sdk-29 which is the latest
* Pass `-Clink-self-contained=n` instead of
  `-Ctarget-feature=-crt-static` (which didn't work) to force using the
  external libc.

(backport <#4837>)
(cherry picked from commit 6259f3a)

Author: alexcrichton

ci/docker/wasm32-wasip1/Dockerfile

@@ -10,7 +10,7 @@ RUN /wasi.sh
 # library.
 ENV CARGO_TARGET_WASM32_WASIP1_RUNNER=wasmtime \
     CARGO_TARGET_WASM32_WASIP1_LINKER=/opt/wasi-sdk/bin/clang \
-    CARGO_TARGET_WASM32_WASIP1_RUSTFLAGS="-lwasi-emulated-process-clocks -Ctarget-feature=-crt-static" \
+    CARGO_TARGET_WASM32_WASIP1_RUSTFLAGS="-Clink-self-contained=n -lwasi-emulated-process-clocks" \
     CC_wasm32_wasip1=/opt/wasi-sdk/bin/clang \
     CFLAGS_wasm32_wasip1=-D_WASI_EMULATED_PROCESS_CLOCKS \
     PATH=$PATH:/rust/bin:/wasmtime

ci/docker/wasm32-wasip2/Dockerfile

@@ -9,7 +9,7 @@ RUN /wasi.sh
 # itself, this should be fixed upstream.
 ENV CARGO_TARGET_WASM32_WASIP2_RUNNER=wasmtime \
     CARGO_TARGET_WASM32_WASIP2_LINKER=/opt/wasi-sdk/bin/clang \
-    CARGO_TARGET_WASM32_WASIP2_RUSTFLAGS="-lwasi-emulated-process-clocks -Ctarget-feature=-crt-static -Clink-arg=-Wl,--export,cabi_realloc" \
+    CARGO_TARGET_WASM32_WASIP2_RUSTFLAGS="-Clink-self-contained=n -lwasi-emulated-process-clocks -Clink-arg=-Wl,--export,cabi_realloc" \
     CC_wasm32_wasip2=/opt/wasi-sdk/bin/clang \
     CFLAGS_wasm32_wasip2=-D_WASI_EMULATED_PROCESS_CLOCKS \
     PATH=$PATH:/rust/bin:/wasmtime

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=38.0.2
-wasi_sdk=27
+wasi_sdk=29
 
 curl -L https://github.com/bytecodealliance/wasmtime/releases/download/v$wasmtime/wasmtime-v$wasmtime-x86_64-linux.tar.xz |
     tar xJf -