ShellPkg/Application: Fix ">v" cannot update environment variable

ShellPkg/Application: Fix ">v" cannot update environment variable

When ">v" is used to redirect the command output to environment
variable (e.g.: "echo xxx >v yyy"), we only called SetVariable()
to update the variable storage but forgot to update the cached
environment variables in gShellEnvVarList.
When updating the variable storage, the existing code unnecessary
saved the ending NULL character into variable storage.

The patch fixes all the above issues.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Chen A Chen <chen.a.chen@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Tapan Shah <tapandshah@hpe.com>