buildRustPackage: dont rely on NIX_BUILD_TOP in cargoSetupPostPatchHook

This breaks the builder when a nix-shell or keepBuildTree is used. The
issue occurs because paths to cargo lockfiles are read with NIX_BUILD_TOP,
which is not reliable.

This breaks a nix-shell because NIX_BUILD_TOP simply is not set, causing
an invalid path to be used. This can be worked around using
NIX_BUILD_TOP=$PWD, but that obviously is not great.

This breaks keepBuildTree because it changes the working directory to a
different path than NIX_BUILD_TOP. Since the lockfiles are copied based
on the working directory, but read based on NIX_BUILD_TOP, this causes
the hook to not be able to find them.

This was solved by both reading these files based on the working directory,
using absolute paths to avoid having to traverse back in the directory tree.

Fixes: #138554

pkgs/build-support/rust/hooks/cargo-setup-hook.sh

@@ -7,23 +7,23 @@ cargoSetupPostUnpackHook() {
     # this for us automatically.
     if [ -z $cargoVendorDir ]; then
         unpackFile "$cargoDeps"
-        export cargoDepsCopy=$(stripHash $cargoDeps)
+        export cargoDepsCopy="$(realpath "$(stripHash $cargoDeps)")"
     else
-      cargoDepsCopy="$sourceRoot/${cargoRoot:+$cargoRoot/}${cargoVendorDir}"
+        cargoDepsCopy="$(realpath "$(pwd)/$sourceRoot/${cargoRoot:+$cargoRoot/}${cargoVendorDir}")"
     fi
 
     if [ ! -d .cargo ]; then
         mkdir .cargo
     fi
 
-    config="$(pwd)/$cargoDepsCopy/.cargo/config";
+    config="$cargoDepsCopy/.cargo/config";
     if [[ ! -e $config ]]; then
       config=@defaultConfig@
     fi;
 
     tmp_config=$(mktemp)
     substitute $config $tmp_config \
-      --subst-var-by vendor "$(pwd)/$cargoDepsCopy"
+      --subst-var-by vendor "$cargoDepsCopy"
     cat ${tmp_config} >> .cargo/config
 
     cat >> .cargo/config <<'EOF'
@@ -39,8 +39,8 @@ EOF
 cargoSetupPostPatchHook() {
     echo "Executing cargoSetupPostPatchHook"
 
-    cargoDepsLockfile="$NIX_BUILD_TOP/$cargoDepsCopy/Cargo.lock"
-    srcLockfile="$NIX_BUILD_TOP/$sourceRoot/${cargoRoot:+$cargoRoot/}/Cargo.lock"
+    cargoDepsLockfile="$cargoDepsCopy/Cargo.lock"
+    srcLockfile="$(pwd)/${cargoRoot:+$cargoRoot/}Cargo.lock"
 
     echo "Validating consistency between $srcLockfile and $cargoDepsLockfile"
     if ! @diff@ $srcLockfile $cargoDepsLockfile; then