Use relative paths in build-details.json

[geofft]

Fixes #712.

[zanieb]

Can you share a diff of the build details?

[geofft]

$ diff -u python{.old,}/install/lib/python3.14t/build-details.json
--- python.old/install/lib/python3.14t/build-details.json	2023-12-31 19:00:00
+++ python/install/lib/python3.14t/build-details.json	2023-12-31 19:00:00
@@ -1,7 +1,7 @@
 {
   "schema_version": "1.0",
-  "base_prefix": "/install",
-  "base_interpreter": "/install/bin/python3.14",
+  "base_prefix": "../..",
+  "base_interpreter": "./bin/python3.14",
   "platform": "linux-s390x",
   "language": {
     "version": "3.14",
@@ -46,12 +46,12 @@
     ]
   },
   "libpython": {
-    "dynamic": "/install/lib/libpython3.14td.so",
+    "dynamic": "./lib/libpython3.14td.so",
     "link_extensions": false,
-    "static": "/install/lib/libpython3.14td.a"
+    "static": "./lib/libpython3.14td.a"
   },
   "c_api": {
-    "headers": "/install/include/python3.14td",
-    "pkgconfig_path": "/install/lib/pkgconfig"
+    "headers": "./include/python3.14td",
+    "pkgconfig_path": "./lib/pkgconfig"
   }
 }