set julia_version in manifest during resolve only

(cherry picked from commit 52a2eeb)

Author: IanButterworth

src/Operations.jl

@@ -288,10 +288,13 @@ end
 function resolve_versions!(env::EnvCache, registries::Vector{Registry.RegistryInstance}, pkgs::Vector{PackageSpec}, julia_version)
     # compatibility
     if julia_version !== nothing
+        env.manifest.julia_version = julia_version
         v = intersect(julia_version, get_compat(env.project, "julia"))
         if isempty(v)
             @warn "julia version requirement for project not satisfied" _module=nothing _file=nothing
         end
+    else
+        env.manifest.julia_version = VERSION
     end
     names = Dict{UUID, String}(uuid => stdlib for (uuid, stdlib) in stdlibs())
     # recursive search for packages which are tracking a path

src/Types.jl

@@ -261,7 +261,7 @@ Base.:(==)(t1::PackageEntry, t2::PackageEntry) = t1.name == t2.name &&
 Base.hash(x::PackageEntry, h::UInt) = foldr(hash, [x.name, x.version, x.path, x.pinned, x.repo, x.tree_hash, x.deps, x.uuid], init=h)  # omits `other`
 
 Base.@kwdef mutable struct Manifest
-    julia_version::Union{Nothing,VersionNumber} = Base.VERSION
+    julia_version::Union{Nothing,VersionNumber} = nothing # only set to VERSION when resolving
     manifest_format::VersionNumber = v"2.0.0"
     deps::Dict{UUID,PackageEntry} = Dict{UUID,PackageEntry}()
     other::Dict{String,Any} = Dict{String,Any}()