ci: cleanup build_flags.sh

From coryan@:
```
scratch_flags=(
--scratch-path "/workspace/.build-cache"

)
build_directory_flags=(
--build-path "/workspace/.build"
)
if git rev-parse --show-toplevel >/dev/null 2>&1; then
scratch_flags=(
--scratch-path "$(git rev-parse --show-toplevel)/.build-cache"
)
build_directory_flags=(
--build-path "$(git rev-parse --show-toplevel)/.build"
)
fi
build_flags=(
-Xswiftc -warnings-as-errors
-Xswiftc -Wwarning
-Xswiftc DeprecatedDeclaration
# Use the versions from `Package.resolved`.
# --disable-automatic-resolution
)
build_flags+=("${scratch_flags[@]}")
build_flags+=("${build_directory_flags[@]}")
```

Start by locating build_flags.sh and its callers, then inspect how the shown scratch, build-path, and Swift compiler flags are used in CI. Done when the cleanup is reflected consistently and the existing build commands continue to work.

Written by the indexing model from the issue text.