Use fixed directory to compile python binary.

Fixes #8

WORKSPACE

@@ -3,28 +3,39 @@ workspace(name="py_test")
 load("@bazel_tools//tools/build_defs/repo:git.bzl", "git_repository")
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 
+PY_VERSION = '3.8.3'
+BUILD_DIR = '/tmp/bazel-python-{0}'.format(PY_VERSION)
+
 # Special logic for building python interpreter with OpenSSL from homebrew.
 # See https://devguide.python.org/setup/#macos-and-os-x
 _py_configure = """
 if [[ "$OSTYPE" == "darwin"* ]]; then
-    ./configure --prefix=$(pwd)/bazel_install --with-openssl=$(brew --prefix openssl)
+    cd {0} && ./configure --prefix={0}/bazel_install --with-openssl=$(brew --prefix openssl)
 else
-    ./configure --prefix=$(pwd)/bazel_install
+    cd {0} && ./configure --prefix={0}/bazel_install
 fi
-"""
+""".format(BUILD_DIR)
 
 http_archive(
     name = "python_interpreter",
-    urls = ["https://www.python.org/ftp/python/3.8.3/Python-3.8.3.tar.xz"],
+    urls = [
+        "https://www.python.org/ftp/python/{0}/Python-{0}.tar.xz".format(PY_VERSION),
+    ],
     sha256 = "dfab5ec723c218082fe3d5d7ae17ecbdebffa9a1aea4d64aa3a2ecdd2e795864",
-    strip_prefix = "Python-3.8.3",
+    strip_prefix = "Python-{0}".format(PY_VERSION),
     patch_cmds = [
-        "mkdir $(pwd)/bazel_install",
+        # Create a build directory outside of bazel so we get consistent path in
+        # the generated files. See #8
+        "mkdir -p {0}".format(BUILD_DIR),
+        "cp -r * {0}".format(BUILD_DIR),
+        # Build python.
         _py_configure,
         # Produce deterministic binary by using a fixed build timestamp and
         # running `ar` in deterministic mode. See #7
-        "SOURCE_DATE_EPOCH=0 make -j $(nproc) ARFLAGS='rvD'",
-        "make install",
+        "cd {0} && SOURCE_DATE_EPOCH=0 make -j $(nproc) ARFLAGS='rvD'".format(BUILD_DIR),
+        "cd {0} && make install".format(BUILD_DIR),
+        # Copy the contents of the build directory back into bazel.
+        "rm -rf * && mv {0}/* .".format(BUILD_DIR),
         "ln -s bazel_install/bin/python3 python_bin",
     ],
     build_file_content = """