SCons: Don't activate `fast_unsafe` automatically on `dev_build`

We experienced first hand why it's called unsafe, and so we should leave it
as an explicit choice for contributors, informing themselves of the caveats.

See #111408.

SConstruct

@@ -210,7 +210,7 @@ opts.Add(
     )
 )
 opts.Add(BoolVariable("tests", "Build the unit tests", False))
-opts.Add(BoolVariable("fast_unsafe", "Enable unsafe options for faster rebuilds", False))
+opts.Add(BoolVariable("fast_unsafe", "Enable unsafe options for faster incremental builds", False))
 opts.Add(BoolVariable("ninja", "Use the ninja backend for faster rebuilds", False))
 opts.Add(BoolVariable("ninja_auto_run", "Run ninja automatically after generating the ninja file", True))
 opts.Add("ninja_file", "Path to the generated ninja file", "build.ninja")
@@ -520,10 +520,10 @@ env.Decider("MD5-timestamp")
 
 # SCons speed optimization controlled by the `fast_unsafe` option, which provide
 # more than 10 s speed up for incremental rebuilds.
-# Unsafe as they reduce the certainty of rebuilding all changed files, so it's
-# enabled by default for `debug` builds, and can be overridden from command line.
+# Unsafe as they reduce the certainty of rebuilding all changed files.
+# If you use it and run into corrupted incremental builds, try to turn it off.
 # Ref: https://github.com/SCons/scons/wiki/GoFastButton
-if methods.get_cmdline_bool("fast_unsafe", env.dev_build):
+if env["fast_unsafe"]:
     env.SetOption("implicit_cache", 1)
     env.SetOption("max_drift", 60)