Fix errors found by running spack audit externals (#47308)

The problem was that `+` is part of the regex grammar, so it needs to be
escaped.

var/spack/repos/builtin/packages/gcc/detection_test.yaml

@@ -26,7 +26,7 @@ paths:
     extra_attributes:
       compilers:
         c: ".*/bin/gcc"
-        cxx: ".*/bin/g++"
+        cxx: ".*/bin/g\\+\\+"
 
 # Mock a version < 7 of GCC that requires -dumpversion and
 # errors with -dumpfullversion

var/spack/repos/builtin/packages/nvhpc/detection_test.yaml

@@ -24,7 +24,7 @@ paths:
     extra_attributes:
       compilers:
         c: ".*/bin/nvc"
-        cxx: ".*/bin/nvc++"
+        cxx: ".*/bin/nvc\\+\\+"
         fortran: ".*/bin/nvfortran"
 - layout:
   - executables:
@@ -51,7 +51,7 @@ paths:
     extra_attributes:
       compilers:
         c: ".*/bin/nvc"
-        cxx: ".*/bin/nvc++"
+        cxx: ".*/bin/nvc\\+\\+"
         fortran: ".*/bin/nvfortran"
 - layout:
   - executables:
@@ -78,5 +78,5 @@ paths:
     extra_attributes:
       compilers:
         c: ".*/bin/nvc"
-        cxx: ".*/bin/nvc++"
+        cxx: ".*/bin/nvc\\+\\+"
         fortran: ".*/bin/nvfortran"