Support using Golang release candidates when install_std is set exp…

…licitly (#266)

* Support using Golang release candidates when `install_std` is set explicitly

`semver_check` cannot handle Go release candidate versions like
`1.23rc2`, and so the if statement here would produce an error like:

```
plz-out/gen/go/build_defs/go.build_defs:48:26: error: failed to parse version: Invalid Semantic Version

    if version != "" and semver_check(version, ">= 1.20.0") and install_std is None:
```

This moves the check for `install_std` being explicitly set to before
the semver check, which allows skipping it.

* Make nullness check first, and strip off the RC for checking the version constraint

ChangeLog

@@ -1,3 +1,7 @@
+Version 1.17.5
+--------------
+    * Support using Golang release candidates when `install_std` is set explicitly
+
 Version 1.17.4
 --------------
     * Prevent modfile from being exposed to downstream actions

VERSION

@@ -1 +1 @@
-1.17.4
+1.17.5

build_defs/go.build_defs

@@ -45,7 +45,7 @@ def go_toolchain(name:str, url:str|dict = '', version:str = '', hashes:list = []
         hashes = hashes,
     )
 
-    if version != "" and semver_check(version, ">= 1.20.0") and install_std is None:
+    if install_std is None and version != "" and semver_check(_strip_version_rc(version), ">= 1.20.0"):
         install_std = True
 
     return _go_toolchain(
@@ -1804,3 +1804,7 @@ def _stdlib():
             stdlib = stdlib.removeprefix('///')
         stdlib = f'///{CONFIG.OS}_{CONFIG.ARCH}{stdlib}'
     return [stdlib]
+
+def _strip_version_rc(version):
+    """Removes an rcX suffix from a version string."""
+    return version.split('rc')[0]