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ARROW-17050: [CI] Use -y flag on mamba install to not ask for confirmation (#13579)

When locally building the `conda-cpp-valgrind` docker image I've realised we are prompting for confirmation:

```
#10 12.35 Confirm changes: [Y/n] 
#10 88.01  
```
We should use the -y flag when using mamba install:
```
  -y, --yes             Do not ask for confirmation.
```
With this change we are no longer seeing the `Confirm changes` message from mamba.

Authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>