depends: Drop trailing slash from `CMAKE_INSTALL_LIBDIR` #35463

1. The trailing slash is redundant.
2. It causes CMP0177-specifc warnings when using CMake >=3.31.

On the master branch:

$ gmake -C depends freetype_configured
gmake: Entering directory '/home/hebasto/dev/bitcoin/depends'
Extracting freetype...
/home/hebasto/dev/bitcoin/depends/sources/freetype-2.11.1.tar.gz: OK
Preprocessing freetype...
patching file CMakeLists.txt
Configuring freetype...
-- The C compiler identification is GNU 15.2.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/gcc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Looking for unistd.h
-- Looking for unistd.h - found
-- Looking for fcntl.h
-- Looking for fcntl.h - found
CMake Warning (dev) at CMakeLists.txt:577 (install):
  Policy CMP0177 is not set: install() DESTINATION paths are normalized.  Run
  "cmake --help-policy CMP0177" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.
This warning is for project developers.  Use -Wno-dev to suppress it.

CMake Warning (dev) at CMakeLists.txt:596 (install):
  Policy CMP0177 is not set: install() DESTINATION paths are normalized.  Run
  "cmake --help-policy CMP0177" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.
This warning is for project developers.  Use -Wno-dev to suppress it.

CMake Warning (dev) at CMakeLists.txt:601 (install):
  Policy CMP0177 is not set: install() DESTINATION paths are normalized.  Run
  "cmake --help-policy CMP0177" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.
This warning is for project developers.  Use -Wno-dev to suppress it.

-- Configuring done (0.2s)
-- Generating done (0.0s)
CMake Warning:
  Manually-specified variables were not used by the project:

    CMAKE_EXPORT_NO_PACKAGE_REGISTRY


-- Build files have been written to: /home/hebasto/dev/bitcoin/depends/work/build/x86_64-pc-linux-gnu/freetype/2.11.1-0539b8d2229/build
gmake: Leaving directory '/home/hebasto/dev/bitcoin/depends'

With this PR:

$ gmake -C depends freetype_configured
gmake: Entering directory '/home/hebasto/dev/bitcoin/depends'
Extracting freetype...
/home/hebasto/dev/bitcoin/depends/sources/freetype-2.11.1.tar.gz: OK
Preprocessing freetype...
patching file CMakeLists.txt
Configuring freetype...
-- The C compiler identification is GNU 15.2.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/gcc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Looking for unistd.h
-- Looking for unistd.h - found
-- Looking for fcntl.h
-- Looking for fcntl.h - found
-- Configuring done (0.2s)
-- Generating done (0.0s)
CMake Warning:
  Manually-specified variables were not used by the project:

    CMAKE_EXPORT_NO_PACKAGE_REGISTRY


-- Build files have been written to: /home/hebasto/dev/bitcoin/depends/work/build/x86_64-pc-linux-gnu/freetype/2.11.1-c25288632f9/build
gmake: Leaving directory '/home/hebasto/dev/bitcoin/depends'

<!--e57a25ab6845829454e8d69fc972939a-->

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

<!--006a51241073e994b41acfe9ec718e94-->

Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35463.

<!--021abf342d371248e50ceaed478a90ca-->

Reviews

See the guideline for information on the review process. A summary of reviews will appear here.

<!--5faf32d7da4f0f540f40219e4f7537a3-->