cabal-install: Fix the directory the build folder is created in

Clearly this path needs to be relative to the CWD, and the correct
offset is passed as the `srcdir` parameter.

Fixes haskell#10772

Author: mpickering

cabal-install/src/Distribution/Client/ProjectBuilding/UnpackedPackage.hs

@@ -459,7 +459,7 @@ buildInplaceUnpackedPackage
     -- TODO: [code cleanup] there is duplication between the
     --      distdirlayout and the builddir here builddir is not
     --      enough, we also need the per-package cachedir
-    createDirectoryIfMissingVerbose verbosity True $ getSymbolicPath builddir
+    createDirectoryIfMissingVerbose verbosity True $ interpretSymbolicPath (Just srcdir) builddir
     createDirectoryIfMissingVerbose
       verbosity
       True

changelog.d/pr-10800

@@ -0,0 +1,12 @@
+synopsis: Fix regression where 'build' folder was created in CWD when running a standlone script.
+packages: cabal-install
+prs: #10800
+issues: #10772
+significance:
+
+description: {
+
+Fix a regression where the `build` folder was created in the current directory
+when running a standalone script.
+
+}