Consume versioned PEX releases (#260)

See pantsbuild/pants#19683 for the related
Pantsbuild automation change.

This change makes it so we now consume PEX names with the expected Pants
version in them.

This change isn't guarded by version, as I expect to backport this
change to earlier releases.

tools/src/scie_pants/install_pants.py

@@ -82,11 +82,11 @@ def install_pants_from_pex(
     """Installs Pants into the venv using the platform-specific pre-built PEX."""
     uname = os.uname()
     major, minor = sys.version_info[:2]
-    pex_name = f"pants.cp{major}{minor}-{uname.sysname.lower()}_{uname.machine.lower()}.pex"
+    pex_name = (
+        f"pants.{version}-cp{major}{minor}-{uname.sysname.lower()}_{uname.machine.lower()}.pex"
+    )
+    pex_url = f"https://github.com/pantsbuild/pants/releases/download/release_{version}/{pex_name}"
     with tempfile.NamedTemporaryFile(suffix=".pex") as pants_pex:
-        pex_url = (
-            f"https://github.com/pantsbuild/pants/releases/download/release_{version}/{pex_name}"
-        )
         try:
             ptex.fetch_to_fp(pex_url, pants_pex.file)
         except subprocess.CalledProcessError as e: