look for python2 symlinks before bootstrap python

Before this commit, if you're running x.py directly on a system where
`python` is symlinked to Python 3, then the `python` config option will
default to a Python 3 interpreter. This causes debuginfo tests to fail
with an opaque error message, since they have a hard requirement on
Python 2.

This commit modifies the Python probe behavior to look for python2.7 and
python2 *before* using the interpreter used to execute `x.py`.

config.toml.example

@@ -162,6 +162,9 @@
 # Python interpreter to use for various tasks throughout the build, notably
 # rustdoc tests, the lldb python interpreter, and some dist bits and pieces.
 # Note that Python 2 is currently required.
+#
+# Defaults to python2.7, then python2. If neither executable can be found, then
+# it defaults to the Python interpreter used to execute x.py.
 #python = "python2.7"
 
 # Force Cargo to check that Cargo.lock describes the precise dependency

src/bootstrap/sanity.rs

@@ -109,9 +109,9 @@ pub fn check(build: &mut Build) {
     }
 
     build.config.python = build.config.python.take().map(|p| cmd_finder.must_have(p))
-        .or_else(|| env::var_os("BOOTSTRAP_PYTHON").map(PathBuf::from)) // set by bootstrap.py
         .or_else(|| cmd_finder.maybe_have("python2.7"))
         .or_else(|| cmd_finder.maybe_have("python2"))
+        .or_else(|| env::var_os("BOOTSTRAP_PYTHON").map(PathBuf::from)) // set by bootstrap.py
         .or_else(|| Some(cmd_finder.must_have("python")));
 
     build.config.nodejs = build.config.nodejs.take().map(|p| cmd_finder.must_have(p))