[bazel] Make pip wheels install reproducible

When installing python packages using pip, we use a custom script
that invokes python directly. By default, python will timestamp
bytecode files hence making the builds non-reproducible which is
bad for CI. Python also uses a random seed for hash.
This commit sets two environment variables when running pip that
force python to use a deterministic seed and to use a fixed
timestamp instead of the current time.

Signed-off-by: Amaury Pouly <amaury.pouly@lowrisc.org>

third_party/python/pip.bzl

@@ -46,6 +46,10 @@ def _pip_wheel_impl(rctx):
     rctx.report_progress("Installing the Python wheel package")
     result = rctx.execute(
         args,
+        environment = {
+            "SOURCE_DATE_EPOCH": "315532800",
+            "PYTHONHASHSEED": "0",
+        },
         timeout = rctx.attr.timeout,
         quiet = rctx.attr.quiet,
     )
@@ -66,6 +70,10 @@ def _pip_wheel_impl(rctx):
     rctx.report_progress("Pre-building Python wheels")
     result = rctx.execute(
         args,
+        environment = {
+            "SOURCE_DATE_EPOCH": "315532800",
+            "PYTHONHASHSEED": "0",
+        },
         timeout = rctx.attr.timeout,
         quiet = rctx.attr.quiet,
     )
@@ -81,6 +89,10 @@ def _pip_wheel_impl(rctx):
     rctx.report_progress("Generating sanitzed requirements file")
     result = rctx.execute(
         args,
+        environment = {
+            "SOURCE_DATE_EPOCH": "315532800",
+            "PYTHONHASHSEED": "0",
+        },
         timeout = rctx.attr.timeout,
         quiet = rctx.attr.quiet,
         working_directory = "./",