Cargo: update ostree-ext to b76d75d6

This updates ostree-ext and related dependencies (composefs-rs,
bootc-internal-utils) to address an issue where G_MESSAGES_DEBUG=all
causes packages to be incorrectly shown as "Removed" during rebase
operations.

The root cause was that GLib debug output was being written to stdout
instead of stderr, corrupting the ostree commit hash parsing in
ostree-ext's container layer import. When the subprocess output is
polluted with debug messages, the merge commit ends up missing content
from derived layers.

Fixes: OCPBUGS-64692
Fixes: RHEL-130454
See: bootc-dev/bootc#1917

Author: jmarrero

Cargo.lock

@@ -73,7 +73,7 @@ openssl = "0.10.73"
 once_cell = "1.21.3"
 os-release = "0.1.0"
 # We pull this one from git, as the project is no longer published as an external crate.
-ostree-ext = { git = "https://github.com/containers/bootc", rev = "40ebdd5829a151cadb5dbadcb1b047f8f4d038cb" }
+ostree-ext = { git = "https://github.com/containers/bootc", rev = "b76d75d6024bd0aa0f270ff181807aff4209f9c9" }
 pastey = "0.1.1"
 phf = { version = "0.12", features = ["macros"] }
 rand = "0.9.1"