Suppress color in cargo tree warnings for simpler grepping

Without this, the `check` recipe in the `justfile` was failing on
CI due to the change in the previous commit. On CI, we set
`CARGO_TERM_COLOR` to `always`. The `--color` option takes
precedence over the `CARGO_TERM_COLOR` environment variable.

The suppresion is only done on those specific command whose
stderr is being parsed. Other colorization remains allowed,
including on CI.

(An alternative to using `--color` here could be to make the `grep`
pattern tolerate color codes where they may be present. But this
would make the pattern considerably longer, more complicated, less
less readable, and easier to get wrong.)

Author: EliahKagan

justfile

@@ -51,13 +51,13 @@ check:
     ! cargo check -p gix-transport --all-features 2>/dev/null
     ! cargo check -p gix-protocol --all-features 2>/dev/null
     # warning happens if nothing found, no exit code :/
-    cargo tree -p gix --no-default-features -e normal -i imara-diff \
+    cargo --color=never tree -p gix --no-default-features -e normal -i imara-diff \
         2>&1 >/dev/null | grep '^warning: nothing to print\>'
-    cargo tree -p gix --no-default-features -e normal -i gix-submodule \
+    cargo --color=never tree -p gix --no-default-features -e normal -i gix-submodule \
         2>&1 >/dev/null | grep '^warning: nothing to print\>'
-    cargo tree -p gix --no-default-features -e normal -i gix-pathspec \
+    cargo --color=never tree -p gix --no-default-features -e normal -i gix-pathspec \
         2>&1 >/dev/null | grep '^warning: nothing to print\>'
-    cargo tree -p gix --no-default-features -e normal -i gix-filter \
+    cargo --color=never tree -p gix --no-default-features -e normal -i gix-filter \
         2>&1 >/dev/null | grep '^warning: nothing to print\>'
     ! cargo tree -p gix --no-default-features -i gix-credentials 2>/dev/null
     cargo check --no-default-features --features lean