CI: take advantage of zig build concurrency

I ain't afraid of no ghost.

This reverts commit 14a176b9b16e07a66a2f9cd485aaf80fed0f5a12.

ci/aarch64-linux-debug.sh

@@ -60,7 +60,6 @@ stage3-debug/bin/zig build -Dtarget=arm-linux-musleabihf
 # TODO: add -fqemu back to this line
 
 stage3-debug/bin/zig build test docs \
-  -j1 \
   -fwasmtime \
   -Dstatic-llvm \
   -Dtarget=native-native-musl \

ci/aarch64-linux-release.sh

@@ -60,7 +60,6 @@ stage3-release/bin/zig build -Dtarget=arm-linux-musleabihf
 # TODO: add -fqemu back to this line
 
 stage3-release/bin/zig build test docs \
-  -j1 \
   -fwasmtime \
   -Dstatic-llvm \
   -Dtarget=native-native-musl \

ci/aarch64-macos.sh

@@ -44,7 +44,6 @@ PATH="$HOME/local/bin:$PATH" cmake .. \
 $HOME/local/bin/ninja install
 
 stage3-release/bin/zig build test docs \
-  -j1 \
   --zig-lib-dir "$(pwd)/../lib" \
   -Denable-macos-sdk \
   -Dstatic-llvm \

ci/x86_64-linux-debug.sh

@@ -58,7 +58,6 @@ stage3-debug/bin/zig fmt --check .. \
 stage3-debug/bin/zig build -Dtarget=arm-linux-musleabihf
 
 stage3-debug/bin/zig build test docs \
-  -j1 \
   -fqemu \
   -fwasmtime \
   -Dstatic-llvm \

ci/x86_64-linux-release.sh

@@ -59,7 +59,6 @@ stage3-release/bin/zig fmt --check .. \
 stage3-release/bin/zig build -Dtarget=arm-linux-musleabihf
 
 stage3-release/bin/zig build test docs \
-  -j1 \
   -fqemu \
   -fwasmtime \
   -Dstatic-llvm \

ci/x86_64-macos-debug.sh

@@ -48,7 +48,6 @@ cmake .. \
 make $JOBS install
 
 stage3/bin/zig build test docs \
-  -j1 \
   --zig-lib-dir "$(pwd)/../lib" \
   -Denable-macos-sdk \
   -Dstatic-llvm \

ci/x86_64-macos-release.sh

@@ -48,7 +48,6 @@ cmake .. \
 make $JOBS install
 
 stage3/bin/zig build test docs \
-  -j1 \
   --zig-lib-dir "$(pwd)/../lib" \
   -Denable-macos-sdk \
   -Dstatic-llvm \