gh-37701: Default SAGE_ECMBIN to "ecm".
    
Having `None` as default breaks the feature ecm.  This restores the
previous default, which corresponds to the upstream name for the binary.

See #37011 (comment)

### 📝 Checklist

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
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URL: #37701
Reported by: Gonzalo Tornaría
Reviewer(s): Dima Pasechnik

Author: Release Manager

src/sage/env.py

@@ -220,7 +220,7 @@ def var(key: str, *fallbacks: Optional[str], force: bool = False) -> Optional[st
 MAXIMA_FAS = var("MAXIMA_FAS")
 KENZO_FAS = var("KENZO_FAS")
 SAGE_NAUTY_BINS_PREFIX = var("SAGE_NAUTY_BINS_PREFIX", "")
-SAGE_ECMBIN = var("SAGE_ECMBIN")
+SAGE_ECMBIN = var("SAGE_ECMBIN", "ecm")
 RUBIKS_BINS_PREFIX = var("RUBIKS_BINS_PREFIX", "")
 FOURTITWO_HILBERT = var("FOURTITWO_HILBERT")
 FOURTITWO_MARKOV = var("FOURTITWO_MARKOV")