Clarify unlocked input warning message

The previous message was vague about what "deprecated" meant and why
unlocked inputs with NAR hashes "may not be reproducible". It also
used "verifiable" which was confusing.

The new message makes it clear that the NAR hash provides verification
(is checked by NAR hash) and explicitly states the failure modes:
garbage collection and sharing.

Author: roberth

src/libexpr/primops/fetchTree.cc

@@ -199,8 +199,8 @@ static void fetchTree(
     if (state.settings.pureEval && !input.isLocked()) {
         if (input.getNarHash())
             warn(
-                "Input '%s' is unlocked (e.g. lacks a Git revision) but does have a NAR hash. "
-                "This is deprecated since such inputs are verifiable but may not be reproducible.",
+                "Input '%s' is unlocked (e.g. lacks a Git revision) but is checked by NAR hash. "
+                "This is not reproducible and will break after garbage collection or when shared.",
                 input.to_string());
         else
             state

src/libflake/lockfile.cc

@@ -77,8 +77,8 @@ LockedNode::LockedNode(const fetchers::Settings & fetchSettings, const nlohmann:
     if (!lockedRef.input.isLocked() && !lockedRef.input.isRelative()) {
         if (lockedRef.input.getNarHash())
             warn(
-                "Lock file entry '%s' is unlocked (e.g. lacks a Git revision) but does have a NAR hash. "
-                "This is deprecated since such inputs are verifiable but may not be reproducible.",
+                "Lock file entry '%s' is unlocked (e.g. lacks a Git revision) but is checked by NAR hash. "
+                "This is not reproducible and will break after garbage collection or when shared.",
                 lockedRef.to_string());
         else
             throw Error(