INTEGRATION_schema_test fails for new versions of xmllint

## Environment
* OS Version: 26.04
* Source or binary build? source build of https://github.com/gazebosim/sdformat/pull/1655/commits/76713ad762c697f928e280ed8b783b99c5afea6d from https://github.com/gazebosim/sdformat/pull/1655

## Description
* Expected behavior: INTEGRATION_schema_test should pass
* Actual behavior: INTEGRATION_schema_test fails

## Steps to reproduce

1. Install `xmllint` from `libxml2-utils` on Ubuntu or `libxml2` on brew, with a version at least 2.15.1. On Ubuntu 26.04 and homebrew, the default version is higher than this
2. Compile `sdformat` and run `ctest -VV -R INTEGRATION_schema_test`

## Output

from CI in https://github.com/gazebosim/sdformat/pull/1655: https://github.com/gazebosim/sdformat/actions/runs/25358850325/job/74487225142?pr=1655

~~~
[ RUN ] SDFSchemaGenerator.TestDoublePendulum
I/O warning : failed to load "http://sdformat.org/schemas/types.xsd": No such file or directory
/github/workspace/build/sdf/1.12/root.xsd:8: element include: Schemas parser error : Element '{http://www.w3.org/2001/XMLSchema}include': Failed to load the document 'http://sdformat.org/schemas/types.xsd' for inclusion.
WXS schema /github/workspace/build/sdf/1.12/root.xsd failed to compile
CMD[xmllint --noout --schema /github/workspace/build/sdf/1.12/root.xsd /github/workspace/test/integration/model/double_pendulum.sdf]
/github/workspace/test/integration/schema_test.cc:40: Failure
Failed
Fail in parsing the model
[ FAILED ] SDFSchemaGenerator.TestDoublePendulum (2 ms)
[ RUN ] SDFSchemaGenerator.TestPR2Model
I/O warning : failed to load "http://sdformat.org/schemas/types.xsd": No such file or directory
/github/workspace/build/sdf/1.12/root.xsd:8: element include: Schemas parser error : Element '{http://www.w3.org/2001/XMLSchema}include': Failed to load the document 'http://sdformat.org/schemas/types.xsd' for inclusion.
WXS schema /github/workspace/build/sdf/1.12/root.xsd failed to compile
CMD[xmllint --noout --schema /github/workspace/build/sdf/1.12/root.xsd /github/workspace/test/integration/model/pr2.sdf]
/github/workspace/test/integration/schema_test.cc:40: Failure
Failed
Fail in parsing the model
[ FAILED ] SDFSchemaGenerator.TestPR2Model (1 ms)
[ RUN ] SDFSchemaGenerator.TestTurtleBotModel
I/O warning : failed to load "http://sdformat.org/schemas/types.xsd": No such file or directory
/github/workspace/build/sdf/1.12/root.xsd:8: element include: Schemas parser error : Element '{http://www.w3.org/2001/XMLSchema}include': Failed to load the document 'http://sdformat.org/schemas/types.xsd' for inclusion.
WXS schema /github/workspace/build/sdf/1.12/root.xsd failed to compile
CMD[xmllint --noout --schema /github/workspace/build/sdf/1.12/root.xsd /github/workspace/test/integration/model/turtlebot.sdf]
/github/workspace/test/integration/schema_test.cc:40: Failure
Failed
Fail in parsing the model
[ FAILED ] SDFSchemaGenerator.TestTurtleBotModel (1 ms)
~~~