breakpointHook: support FODs and non-sandboxed builds

Remove the static reference to /build, instead evaluate NIX_BUILD_TOP

Author: DavHau

pkgs/by-name/br/breakpointHook/attach.sh

@@ -56,6 +56,10 @@ elif [ "$(echo "$pids" | wc -l)" -ne 1 ]; then
 fi
 pid="$(echo "$pids" | head -n1)"
 
+
+# get the build top level directory inside the sandbox (eg. /build)
+buildDir=$(getVar NIX_BUILD_TOP)
+
 # bash is needed to load the env vars, as we do not know the syntax of the debug shell.
 # bashInteractive is used instead of bash, as we depend on it anyways, due to it being
 #   the default debug shell
@@ -71,11 +75,11 @@ pwd="$(readlink /proc/$pid/cwd)"
 # If another shell is chosen via `debugShell`, it will only have simple env vars avaialable.
 exec nsenter --mount --ipc --uts --pid  --net --target "$pid" "$bashInteractive" -c "
   set -eu -o pipefail
-  source /build/env-vars
+  source \"$buildDir/env-vars\"
   cd \"$pwd\"
   if [ -n \"$debugShell\" ]; then
     exec \"$debugShell\"
   else
-    exec \"$bashInteractive\" --init-file /build/env-vars
+    exec \"$bashInteractive\" --init-file \"$buildDir/env-vars\"
   fi
 "