fix: Invalid character ('(') in a variable name: 'PROGRAMFILES'

[lukka]

CMake Warning (dev) at ..../vcpkg/scripts/buildsystems/vcpkg.cmake:133 (file):
Policy CMP0053 is not set: Simplify variable reference and escape sequence
evaluation. Run "cmake --help-policy CMP0053" for policy details. Use the
cmake_policy command to set the policy and suppress this warning.

For input:

'$ENV{PROGRAMFILES(x86)}'

the old evaluation rules produce:

'C:\Program Files (x86)'

but the new evaluation rules produce an error:

Syntax error in cmake code at
  ..../vcpkg/scripts/buildsystems/vcpkg.cmake:133
when parsing string
  $ENV{PROGRAMFILES(x86)}
Invalid character ('(') in a variable name: 'PROGRAMFILES'

Using the old result for compatibility since the policy is not set.

[Rastaban]

Ug, I am going to go ahead an merge this to unbreak master even though modifications to this file does not force any real tests to run in the CI system.