nix: Refactor checkedShellScript's inRootDir to workingDir

This allows more flexible control over the working directory. Values for workingDir must always start
with a / and will then be relative to the repo root.

nix/tools/loadtest.nix

@@ -43,7 +43,7 @@ let
           "ARG_OPTIONAL_SINGLE([testdir], [t], [Directory to load tests and fixtures from], [./test/load])"
           "ARG_LEFTOVERS([additional vegeta arguments])"
         ];
-        inRootDir = true;
+        workingDir = "/";
       }
       ''
         # previously required settings to make this work with older branches
@@ -90,7 +90,7 @@ let
               __gitcomp_nl "$(__git_refs)"
             fi
           '';
-        inRootDir = true;
+        workingDir = "/";
       }
       ''
         for tgt in "''${_arg_target[@]}"; do
@@ -140,7 +140,7 @@ let
           "ARG_POSITIONAL_SINGLE([file], [Filename of result to create report for])"
           "ARG_LEFTOVERS([additional vegeta arguments])"
         ];
-        inRootDir = true;
+        workingDir = "/";
       }
       ''
         ${vegeta}/bin/vegeta report -type=json "$_arg_file" \
@@ -169,7 +169,7 @@ let
       {
         name = "postgrest-loadtest-report";
         docs = "Create a report of all loadtest reports as markdown.";
-        inRootDir = true;
+        workingDir = "/";
       }
       ''
         find loadtest -type f -iname '*.bin' -exec ${reporter} {} \; \