use PYTHON_SYS_EXECUTABLE for setting path to Python

.bazelrc

@@ -1,8 +1,9 @@
 common --enable_platform_specific_config
 common --experimental_repository_cache_hardlinks
 
-# specify python path on Windows for pyo3 compile
+# specify python path for pyo3 compile
 build:windows --action_env="PYTHON_SYS_EXECUTABLE=c:\\python\\python.exe"
+#build:linux --action_env="PYTHON_SYS_EXECUTABLE=/usr/local/bin/python3.8"
 
 # runfiles are off by default on Windows, and we need them
 build --enable_runfiles

docs/linux.md

@@ -31,7 +31,7 @@ other than /usr/bin, you'll need to put the following into a file called user.ba
 at the top of this repo before proceeding:
 
 ```
-build --action_env=PYTHON_SYS_EXECUTABLE=/usr/local/bin/python
+build --action_env=PYTHON_SYS_EXECUTABLE=/usr/local/bin/python3.8
 ```
 
 If you're building Anki from a docker container or distro that has no `python` command in

python.bzl

@@ -8,13 +8,16 @@ def _impl(rctx):
         "python.exe",
     ]
     path = None
-    for name in names:
-        path = rctx.which(name)
-        if path:
-            break
+    if rctx.os.environ.get("PYTHON_SYS_EXECUTABLE"):
+        path = rctx.os.environ.get("PYTHON_SYS_EXECUTABLE")
+    else:
+        for name in names:
+            path = rctx.which(name)
+            if path:
+                break
 
     if not path:
-        fail("python3 or python.exe not found on path")
+        fail("python3 or python.exe not found on path, and PYTHON_SYS_EXECUTABLE not set")
 
     rctx.symlink(path, "python")
     rctx.file("BUILD.bazel", """